In this paper the author presents a method to induce passive cutaneous anaphylaxis of guinea pigs to penicillin and discusses the mechanism of antigenic binding of penicillin to albumin. Penicillin G and recrystallized egg white albumin were incubated at 37℃ for 24 hours. The precipitates obtained by incubation were centrifuged and desolved in buffered saline (pH 7.6 to 7.8). All rabbit antisera of 8 to 10 weeks sensitization by these penicillin-albumin conjugated yielded high titer (1,000 times dilution) of P.C.A. reaction. Antigenic properties of penicillin were simple hapten as observed in the ileum contraction test of actively sensitized guinea pigs. As far the mechanism of antigenic binding of penicillin to albumin, after incubation of penicillin G and albumin at 37℃ for 24 hours, the pH of the medium decreased from 6.2 to 4.6. By making the medium alkaline the P.C.A. titer was almost completely lost. On the contrary, by acidification the binding process was accelerated. The incubated mixture of penicillin G and tissue homogenates such as liver, spleen, lung and skin did not induce P.C.A., with the exception of the exsudates, which usually have acid reaction. Normal serum incubated with penicillin G gave no positive result, but after acidification and dialysis positive reaction was observed. Benzyl penicillenic acid, a intermediate degradation product of penicillin G was reported to be a chemically active substance, which binds easily certain aminoacid residures of protein molecule. Benzyl penicillenic acid was measured by the optical absorption at 322 mμ. The pH effects on the antigenic binding of penicillin G to albumin were the same on the conversion rate of penicillin G to benzyl penicillenic acid. Therfore irreversible binding of penicillin G to albumin to make a complete antigen appeared to be mediated by the formation of benzyl penicillenic acid. However most of the sera from patients with anaphylactoid reaction to penicillin, sometime to other drugs, irrespective to reagin to penicillin had strong capacity for antigenic binding without acidification. The mechanism of this type of antigenic binding of penicillin to protein remained to be studied.