Sera from two patients with granulocytopenia associated with collagen vascular disease caused the destruction of normal human granulocytes by autologous lymphocytes in vitro. Granulocyte cytotoxicity was measured by the release of 51Cr during incubation with test sera and lymphocytes in microtiter plates. Between 8% and 46% granulocytoxicity was produced in granulocytes from 8 normal donors by the sera from these two patients. Less than 6% granulocytotoxicity was seen with the sera from 14 normal subjects and 29 patient controls. Treatment of lymphocyte preparations with carbonyl iron and magnetic separation to remove phagocytic cells or treatment with complement-coated red cells followed by repeated gradient centrifugation to remove complement receptor- bearing lymphocytes did not reduce the granulocytotoxicity. There was a dose-response relationship between the concentration of positive sera and granulocytotoxicity. When these sera were fractionated by Sephadex G-200 gel filtration and by ion-exchange chromatography with DEAE- cellulose, the active component appeared in the IgG-containing fractions. Thus, IgG antibody-dependent, lymphocyte-mediated granulocyte cytotoxicity represents a means of detecting human granulocyte antibodies and is a possible mechanism of autoimmune neutropenia in these two patients.