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CORRESPONDENCE is still quite unknown. The usually accepted figure of 2 mg./ 100 ml. is, in fact, based on in vitro work by Lehmann.3 He showed that the minimum inhibitory concentration (M.I.C.) for ditTerent strains of tubercle bacilli ranged from 0.15 to 1.53 mg./l 00 ml. Btit it has becn found subsequently that the M.I.C. for P.A.S. varies greatly, depending upon the medium on which the bacilli are grown, the size of the inoculum, and the presence or absence of plasma. It is not possible to relate tfhe M.l.C. for P.A.S. in vitro to the minimal serum level of P.A.S. needed to prcvent the emergence of bacillary resistance. Schlenker et al.4 showed that on a total daily dose of 12 g. of calcium B-P.A.S. the blood level of free I'.A.S. seldom exceeds 0.4 mg./100 ml. If this mntichi-quoted, free P.A.S. serum-concentration of 2 mg./100 ml. is valid, then B-P.A.S. therapy with isoniazid would be nothing short of disastrous, for the emergence of isoniazid resistance wotild be commonplace. We very much doubt if it is necessary to give as muich as 10 g. P.A.S. a day when treating patients with a P.A.S.isoniazid combination. Bly the time that most patients are put on to this dual tberapy, their cavities are closed and sputum converted. It is a thousand pities that an M.R.C. trial of 51'H (namely. P.A.S. S g. a day plus isoniazid 100 mg. twice a day) was never attempted on a group of patients who, having greatly improved on adequate bed rest with triple chemotherapy, require a further year or so on antituberctilous drugs to heal their disease. If any difference exists between calcium B-P.A.S. and sodium P.A.S. in their ability to prevent the emergence of bacterial resistance to isoniazid, it must be extremely slight and unlikely to be demonstrated clearly except by long-term sttidy of a large nuniber of cases. Such a marginal inferiority of calcium B-P.A.S., if indeed it exists, muist surely be more than compensated by the greater consumption by patients of this far more acceptable drug.-We are, etc., Bow Arrow Hlospital, M. 0. J. GIBSON. Da rtford.

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