Studies were conducted on serum removed from 15 patients before, during, and after, clozapine-induced agranulocytosis. Cytotoxic studies were compared with samples taken from patients during treatment with clozapine who did not develop agranulocytosis or treatment controls (TC); additional controls consisted of allogeneic (NC) and autogeneic serum from apparently normal people. The effect of serum on measurable functions of polymorphonuclear neutrophils (PMNs) taken from normal people was tested. Procedures under study included suppression of post-phagocytosis-induced 14CO2-indicated respiratory burst, as well as ejection of trypan blue by test PMNs. PMNs exposed to active agranulocytosis serum plus complement displayed diminished 14CO2 emission during phagocytosis or failed to eject trypan blue. PMNs exposed to serum of TC and NC continued to function normally as regards 14CO2 emission and trypan blue ejection. Five patients studied before the development of agranulocytosis showed suppressed PMN function, which increased to peak value during agranulocytosis and then disappeared within 40 days of recovery. Similar suppression of colony forming units of granulocytes and macrophages (CFU-GM) was found whenever agranulocytosis serum was included in the marrow culture. The cytotoxic material required complement for its full expression, was not dialysable, was neutralised by anti-IgM serum, and was absorbed by test PMNs. Furthermore, solutions of clozapine or 5 of its metabolites offered no similar suppression of PMN function in vitro after incubation in an aqueous medium or with normal serum. These observations favour development of an immunogenic clone in sensitive people during active treatment with clozapine, which eventually leads to precipitous depletion of PMNs and their precursors. The early appearance of this suppressive substance may offer an early warning for development of agranulocytosis.