Serum drawn from patients during clozapine-induced agranulocytosis was toxic to human polymorphonuclear leukocytes (PMNs). Toxicity was produced by an immunoglobulin fraction, predominantly of the IgM class. The offending drug was not necessary at this stage to produce cytotoxicity. These effects were observed by inhibiting post-phagocytosis glycolysis, by ejection of trypan blue, or by enhanced release of 51Cr from lysed-labeled PMNs. Direct chemical toxicity, produced by clozapine or its metabolites, was tested by similar procedures. At a concentration of 10(-5) M in a colloidal milieu produced by dilution with AB serum, no cytotoxicity was observed; however, in aqueous medium. N-desmethylclozapine was toxic to PMNs and proliferating lymphocytes. Post-recovery serum appeared to be inert, but cytotoxicity was restored by adding clozapine or N-desmethylclozapine to the sensitive patient's serum. At this stage, cytotoxicity as measured by 51Cr release was abrogated by anti-IgG or anti-IgM. These relationships favor an immunologic mechanism that damages peripheral PMNs. Development of colony-forming units-granulocyte (CFU-G) was similarly inhibited in normal marrow cultures by cytotoxic serum alone, whereas no metabolite had such an effect at the same concentration (10(-5) M).