Abstract.10,853 sera, collected from ‘healthy’ blood donors during the period December 1977 to May 1979, were tested for the presence of penicillinase‐sensitive penicillins by a bio‐assay technique usingBacillus subtilisATCC 6633 as test organism. Their presence could be demonstrated in 30 sera (0.27%), at concentrations equivalent to 0.2–4.0 μg/ml of penicillin G.The implications of these findings are discussed, particularly the possibility of their relationship to nonhemolytic adverse reactions to t