The formation of homocytotropic antibodies (IgE) as determined by immunochemical characteristics against penicilloyl in rabbits was shown. The production of such antibodies to azidocilloyl-human serum albumin (AzO₁₀-HSA) in alum was found optimal using 1 mg of antigen in the tested range of 0.01–8 mg. At the lowest dose (0.01 mg) only haemagglutinating but no IgE antibodies were formed. Immunization with azidocilloyl-bovine γ-globulin (AzO₉-BGG) resulted in a slower increase in antibody levels than was caused by AzO₁₀-HSA. Univalent benzylpenicilloyl-ε-aminocaproate or ampicillin given together with the antigen upon immunization decreased the levels of penicilloyl specific IgE and haemagglutinating antibodies, but induced the formation of IgE antibodies against the carrier molecule. Further, the administration of penicilloyl specific IgG antibodies diminished formation of both IgE and haemagglutinating antibodies, but no antibodies specific for the carrier were formed. The usefulness of this animal model for the experimental study of penicillin allergy is discussed.