IN 1943, soon after the demonstration of the treponemicidal action of penicillin by Mahoney and his co-workers,1 a nationwide study was organized in order to determine the therapeutic efficacy of penicillin in syphilis. Publications of the cooperative study have presented the results of treatment of early syphilis with total dosages ranging from 60,000 to 2,400,000 units given intramuscularly in divided doses over four to fifteen day periods.2 The results thus far available indicate that, although penicillin is effective in syphilis, the optimum regimen of treatment has not yet been defined. Investigation has recently been directed toward determining the effect of (a) utilizing large total dosages and (b) prolonging the duration of treatment. The first of these alternatives is considered in the present paper, which reports preliminary results of a schedule employing commercial sodium penicillin in a total dosage of 4,800,000 units in seven and a half days. The