In an effort to characterize the cellular antigens recognized by anti- neutrophil antibodies in autoimmune neutropenia, we studied sera, purified immunoglobulin G (IgG) and isolated F(ab')2 from 70 neutropenic patients suspected of this diagnosis. Anti-neutrophil antibodies were found in the sera of 36 of these patients by either 125I-staph A binding or immunofluorescence cytometric techniques that detected increased binding of patients' IgG to normal neutrophils. Anti- neutrophil antibody positive sera were then evaluated for specific binding to electrophoretically separated neutrophil membrane-associated proteins by immunoblotting. A 43-Kd protein was consistently identified by eight anti-neutrophil antibody positive sera. The specificity of binding to this protein was confirmed with affinity purified IgG and F(ab')2 fragments prepared from these sera. Sera from 20 healthy normal controls and from 22 non-neutropenic, anti-neutrophil antibody negative rheumatoid arthritis patients failed to bind this protein. Separate studies identified the 43-Kd protein as actin. Purified Acanthamoeba actin comigrated with the protein and was specifically bound by anti- neutrophil antibody positive IgG. Moreover, two actin-specific monoclonal antibodies bound to the 43-Kd membrane-associated protein in immunoblots. In addition, a rabbit anti-actin antiserum not only bound to this same 43-Kd protein but also expressed anti-neutrophil antibody activity against normal human neutrophils, as did purified human anti- actin IgG prepared by affinity chromatography from the serum of one of the index patients. These studies indicate that the anti-neutrophil antibodies of certain patients with autoimmune neutropenia include autoantibodies specific for actin. The molecules on the surface of neutrophils, which have actin-like antigenic epitopes and are recognized by these anti-actin antibodies, remain to be characterized.