Studies were conducted on serum samples collected from 15 patients during the course of clozapine-induced agranulocytosis. During acute phases of agranulocytosis, serum was cytotoxic to peripheral polymorphonuclear neutrophils (PMNs), as indicated by complement-dependent suppression of postphagocytosis respiratory burst and by increased retention of trypan blue dye by test PMNs. Cytotoxicity was removed by adsorption with allogeneic PMNs, was attenuated by antibody to immunoglobulin M but not by antibody to immunoglobulin G antigen-binding fragment, was not dialyzable, and was partially removed by 2-mercaptoethanol and dialysis. Three patients in a longitudinal study all had perturbed postphagocytosis respiratory burst 20 days before agranulocytosis developed. In all patients cytotoxicity disappeared less than 40 days after treatment when the offending drug was discontinued. Trypan blue dye reactivity was similar when tested. At 20% of culture medium, all serum samples partially suppressed development of colony-forming units of granulocytes and macrophages (CFU-GM) in marrow cultures. At 40% of culture medium, agranulocytosis serum suppressed CFU-GM completely but did not inhibit development of colony-forming units of erythroblasts (CFU-E) or burst-forming units of erythroblasts (BFU-E). Addition of clozapine alone or to agranulocytosis serum neither enhanced nor suppressed toxicity to peripheral PMNs. Neither clozapine nor its toxic metabolic end-products directly produced equivalent cytotoxicity to cellular function or proliferation at 10(-5)mol/L. Serum from patients given clozapine who did not have agranulocytosis and samples from allogeneic normal subjects were not cytotoxic to test PMNs. Cytotoxicity was specific to granulocyte cell lines because development of CFU-GM was inhibited by agranulocytosis serum, whereas CFU-E and BFU-E were not similarly affected. Further studies are in progress to distinguish between immunogenic and non-immunogenic mechanisms.