• Twelve patients with chronic neutropenia, ranging in age from 7 to 27 months, were studied for circulating antineutrophil autoantibodies. Absolute neutrophil counts ranged from 0 to 500/cu mm. None of the patients was transfused or had a history of prior drug ingestion. Edetic acid-microagglutination was employed to detect leukocyte antibodies. Sera from six of 12 patients contained antineutrophil antibodies, four reacting with neutrophils from the father and two from the mother. Patient sera also reacted with neutrophils of several unrelated normal volunteers. Four of the six patient sera with antineutrophil antibodies also reacted with autologous neutrophils. The duration of neutropenia was seven months in one patient with antibody whose neutropenia resolved. Patients with neutrophil autoantibody did not clinically differ from those without demonstrable antibody. The coexistent fall in leukoagglutinin titer and rise in neutrophil counts in one patient suggested an etiologic role for this antibody. Detection and proper diagnosis have important therapeutic implications.