SummaryThe minimal curative dose of a commercial penicillin in aqueous solution by intramuscular injection 3 times a day for 10 doses in succession was apparently slightly more than 1000 units per kg (totalling 10,000 units) and probably somewhat less than 5000 units per dose (totalling 50,000 units) in the treatment of acute testicular syphilis of rabbits inoculated with the Nichols-Hough strain of T. pallidum. This strain did not show any evidence of acquired resistance or tolerance to penicillin after 3 consecutive passages through the testicles of rabbits treated with subcurative amounts of the compound.