In a recent issue of Science1 it was reported that gonorrhea had been successfully treated with single injections of 100,000 units of commercial penicillin incorporated in a beeswax-peanut oil base. For the past several months we have had an opportunity to investigate the properties of so-called penicillin X,2 sometimes referred to as factor X, or allopenicillin. This material was furnished to us by three manufacturers.3 A small amount of crystalline penicillin X4 was also made available to us. When assayed by the cup-plate method5 the potency of crystalline penicillin X is approximately 900 units per milligram, while crystalline penicillin6 has a potency of 1,650 units per milligram. In addition, in vitro studies (serial dilution) show that penicillin X is more effective than commercial penicillin against a strain of Klebsiella pneumoniae type A and a strain of Bacillus cereus. No difference in effect could be