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the importance of the H-2 locus may reside, not in a

special potency of its antigens, but in its complexity, sothat strains differing at the H-2 locus are likely to differsimultaneously in several antigens. The importance ofthe H-2 locus in mouse, and the HL-A locus in man, is,therefore, interpretable as one consequence of the cumu-lative effect of all antigens.Adopting a quantitative approach, one considers

potential donors (related or not) as forming a continuousdistribution of degrees of compatibility with a recipient.The shape of this distribution will depend upon the geneticmodel one adopts.In the accompanying figure, Y, the percentage of donors

with an incompatibility not more than X, is plottedagainst X, which is expressed as a continuous variable from0 to 100. The four sets of graphs represent four geneticmodels which I have considered in detail elsewhere.’ 7

The graphs in the figure can be used to predict theclinical success-rate for kidney grafts. If the success of agraft depends upon the incompatibility being not morethan a certain threshold Xt, the success rate will be Yt.Even without immunosuppression one would expect Xtto have a small but finite value. Immunosuppression wouldincrease the clinically acceptable threshold Xt towards100, a limit which it would be difficult to achieve withoutkilling the recipient. The slopes of the curves show theimprovement in survival expected for a given increase inimmunosuppression.The true situation in man is probably a combination

of models n (numerous 2-allele loci) and iv (the HL-Alocus). The models allow us to make certain predictions.

(a) In the absence of immunosuppression sibs make the bestdonors.

(b) With immunosuppression increasing up to 25% of thetotal range the prognosis when the donors are parents will becomeas good as with sib donors.

(c) As immunosuppression increases further still (50% ofthe total range), unrelated donors will begin to show goodprognosis, but related donors will be approaching 100%success-rates.

These models assume no matching of donors and

recipients. But with matching the models can still be usedto represent the residual (unmatched and undetected)incompatibility.

A. J. BATEMAN.

Paterson Laboratories,Christie Hospital andHolt Radium Institute,

Manchester.

P.A.S. IN THE PAN

SIR,-Domestic bleaching agents which contain hypo-chlorite can cause a colour reaction with the urine ofpatients who are taking certain drugs, and perhaps not allsuch reactions have yet been noticed. Cardwell 8 hasdescribed in your columns how a patient taking methyldopawas investigated for haematuria before the explanation forhis red urine became clear. But although it can confuse,such a colour reaction once recognised could also beuseful as a check that a patient is taking his medicationas prescribed.A man taking p-aminosalicylic acid (P.A.s.)-with

isoniazid of course-noticed that his urine turned darkbrown " like blood " in the lavatory pan. Observing(unlike Cardwell’s patient) that the urine stream itselfwas clear, he investigated and found that the colour onlyappeared when a domestic bleaching agent had been putin the pan. (P.A.S. in urine stains lavatory pans yellow andcan lead to the plentiful application of bleach.)

P.A.S. does indeed turn dark brown with a reddish tinge7. Bateman, A. J. Unpublished.8. Cardwell, J. B. Lancet, Aug. 9, 1969, p. 326.

when added to hypochlorite bleach (we used ’ Parazone ’)but salicylic acid does not. A very strong sulphonamidesolution (0-25 g. in a test-tube) turns bright yellow withbleach-quite different from the P.A.S. colour-but urinepassed after taking 2 g. of sulphadimidine by mouth doesnot, though it is shown to contain sulphonamide by theacid-newspaper test.Comparing the reactions of bleach with those of ferric

chloride, we find:

Thus, adding domestic bleach to urine indicates thepresence of P.A.S. more specifically than does adding ferricchloride, and the taking of aspirin does not cause a " falsepositive " reaction. Only an absurdly large amount ofsulphonamide might confuse the issue, and then ferricchloride would exclude it. Diluting the bleach to a 1 in 5solution prevents it fizzing when added to urine, but adefinite colour can still be seen when the P.A.S. contentis as low as 2 mg. per 100 ml. Using the weaker solutionof bleach also prevents the colour being quickly bleachedout again. The bleach reaction is less sensitive thanferric chloride in detecting P.A.s., but indicates its presenceat therapeutic levels and is not mimicked by the taking ofaspirin-though it may be by the taking of methyldopaand perhaps some other drugs which have not yet beennoticed to change colour with hypochlorite.As a corollary, not only must one inquire what medi-

cation a patient is taking when investigating haematuria,but also what type of lavatory cleanser is used at home.’Harpic ’ neither changes colour with P.A.s. nor bleacheslitmus paper.

F. GOULDENJ. C. ANGELL.

Ashford Hospital,Middlesex.

CARE OF THE MENTALLY SUBNORMAL

SiR,ńThe care of the mentally subnormal is underconstant discussion, and Dr. Kushlick has again publishedhis statistics from Wessex (Nov. 29, p. 1196). The ideashe has promulgated have not as yet been tried, and we lookforward to hearing how the Wessex plan is working.Unfortunately costs have to be considered as well as

benefits, and one hopes that when these are published thehospital costs will include the costs of teaching, training,transport, &c.

Hospitals for the mentally subnormal have sufferedfrom severe financial restrictions, and one would like to seehow they would run if they had a reasonable increase ofrevenue, overcrowding were abolished, and staff shortageswere remedied; only in this situation could fair comparisonsbe made. It is also only in these conditions that nursescan fulfil their proper function. They are trained to bepatient-oriented; but, because of overcrowding and staffshortages, we place them in a position where they areunable to use their skills and have to resort to practices(such as Dr. Kushlick describes) which they know to beantitherapeutic.

It would be wonderful to wake up and find that the

community will really accept the mentally handicapped inits midst. We have tried to place mentally subnormalchildren in psediatric units close to their homes, withsingular lack of success. The explanation for the last refusalwas that the matron believed that having a mentally handi-capped child in her children’s ward would be bad for theother children and her staff. We also get letters from the" community " expressing anxiety, and even hostility, overpatients’ being " at large ".

It will take time and a great deal of money to overcomethe years of neglect from which hospital services for the