Antibody responses to penicilloyl were recorded in infants and children treated for about 1 week with intravenous ampicillin. In this single-blind, randomized study two commercially available preparations were compared, one of high-grade purity (< 0.1 μg antigen per gram) and the other slightly contaminated with high molecular weight proteinaceous material (1.5 μg antigen per gram) according to a radioimmunoassay. The results showed no significant increase in the antibody titers in any of the patient groups. The immunogenic properties of high molecular weight proteinaceous impurities isolated from phenoxymethyl penicillin during manufacture were tested in rabbits given daily subcutaneous injections with 0.1–10 μg of the antigen over 10-day periods. When the 10-day period was repeated, the rabbits injected with more than 2 μg of antigen responded with both IgM/IgG and IgE antibodies. The clinical as well as the animal study indicates that antigen impurities in the preparations of the order of 2 μg per gram or less do not elicit a significant antibody response. However, the study in rabbits demonstrates that high molecular weight impurities can induce penicillin allergy if present in about 10-fold higher quantities than those usually found in the commercial penicillin preparations of
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