Antigenicity of penicillin G (PCG) was studied in guinea pigs. PCG 5 mg, 10 mg or 25 mg with Freund's complete adjuvant each on days 0, 7 and 21 was injected to a guinea pig: intramuscularly into both thighs and intracutaneously into four locations on the back. A remarkable antigenicity was induced in animals immunized with 25 mg although only low antigenicity in 5 mg and 10 mg. A maximum serum level of the antibody was observed about 2 weeks after last immunization and all of animals immunized with 25 mg died in active systemic anaphylaxis test. As mentioned above, it has been firstly demonstrated that a remarkable antigenicity of PCG can be produced by immunizing with a high dose of 25 mg in the guinea pig model in which PCG itself is used as immunogen.