1. IgG, IgM and IgE anti‐benzylpenicilloyl (BPO) antibody activities were determined by enzyme‐linked immunosorbent assay (ELISA) in sera from 100 patients who claimed to be allergic to penicillin, and from 50 healthy volunteers. Continuous frequency distributions for all three classes of anti‐BPO antibody, defined as differential binding (delta OD) to BPO‐human serum albumin (HSA) and HSA, were obtained for both groups. 2. For IgM and IgE classes the anti‐BPO activities were slightly but statistically significantly higher in the patient group compared with the volunteer group. 3. Hapten inhibition ELISAs were performed to confirm specificity for the BPO determinant. On the basis of antibody activities (delta OD values) greater than or equal to 0.3 and 50% inhibition of binding in the presence of 100 micrograms ml‐1 BPO‐caproate, BPO‐specific IgG antibody was identified in 4/100 of the patients' sera and in 1/50 of the volunteers' sera; BPO‐specific IgM was identified in 7/100 patients' sera and 1/50 volunteers' sera; and BPO‐specific IgE in 5/100 patients' sera and 1/50 volunteers' sera. 4. Not all sera with differential antibody binding to BPO‐HSA/HSA were inhibited by the BPO hapten. Hence, hapten inhibition assays are essential for the unambiguous demonstration of drug specific antibodies.

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