In an effort to explain the increased incidence of serum sickness-like reactions (SSLR) in patients receiving cefaclor, we used an in vitro murine microsomal system as a surrogate for in vivo hepatic drug biotransformation. Lymphocytes from three groups of subjects were exposed to a nonselective mixture of cefaclor metabolites. After an 18-hour incubation of lymphocytes with these metabolites, cells were examined for viability by trypan blue exclusion. The subject groups consisted of patients with a previous history of SSLR after cefaclor therapy (n = 19), patients who experienced adverse reactions to cefaclor suggestive of immediate hypersensitivity (n = 11), and control subjects who had previously tolerated at least two courses of cefaclor therapy without adverse effect (n = 9). Additionally, immediate family members of six subjects with cefaclor-associated SSLR were studied. Lymphocyte killing was 100% greater than baseline (i.e., a non-drug-containing control) in subjects with SSLR compared with those with immediate hypersensitivity reactions (4% cell death above baseline; p < 0.001) and nonaffected control subjects (6% cell death above baseline; p < 0.001). Family studies were consistent with a pattern of maternal inheritance; five of six mothers who had not received cefaclor had a positive (i.e., > or = 35% cell death above baseline) in vitro cytotoxic response. Other studies confirmed the requirement for biotransformation of the parent drug to elicit cell death, demonstrated specificity of the reaction to cefaclor, illustrated a lack of cross-reactivity to cephalexin in subjects with SSLR to cefaclor, and verified the reproducibility of the reaction over time in an affected subject. Our findings indicate that cefaclor associated SSLR may be a unique adverse drug reaction that requires biotransformation of the parent drug and may result from inherited defects in the metabolism of reactive intermediates. Furthermore, this condition can be retrospectively confirmed with an in vitro lymphocyte-based cytotoxicity assay.