A patient with hyperthyroidism and severe neutropenia had a clinical course and family history that suggested an immune cause. Neutrophil-binding IgG was demonstrated in serum using the Fab-anti Fab assay. The antineutrophil factor bound specifically to either homologous or autologous neutrophils and could be adsorbed by the target neutrophils. The quantity of IgG required to saturate the neutrophil-binding sites (175 000 molecules per neutrophil) and the serum concentration (1.3 mug/ml) were determined. It was estimated that at the time that blood and marrow neutrophils were markedly reduced, serum contained sufficient neutrophil-binding IgG to saturate the binding sites of 1.2 times the total blood neutrophil pool.