A method has previously been described which detected xenogeneic and allogeneic antibodies to human granulocytes by their inhibition of the normal phagocytosis-associated hexose monophosphate shunt (HMS) activity. This method was used to study three patients with acute agranulocytosis secondary to antithyroid drug administration. Two patients with methimazole and one patient with propylthiouracil induced agranulocytosis were studied. Serum samples from each of these three patients taken during the acute phase of agranulocytosis had inhibitory effects on phagocytosis-associated HMS activity in leukocytes from both normal donors and the patients after their full recovery from agranulocytosis. IgM but not IgG prepared from acute sera in two patients was also inhibitory. Disruption of IgM disulfide bonds by dithiothreitol destroyed its inhibitory activity. The possibility of drug-dependent immune destruction of leukocytes in these patients is discussed.