Chronic benign neutropenia of infancy is a disease which develops a few months after birth and which is characterized by a severe selective neutropenia, accompanied by benign but persisting infections. The cause of the disease is still unknown. The sera from 5 such patients were tested for the presence of granulocyte antibodies as a possible cause of the disease. For the detection of these antibodies immunofluorescence, agglutination and granulocytotoxicity were used. All sera contained antibodies which reacted both with the neutrophils of one or both parents of the patient and a part of a panel of unrelated donors. From the reaction patterns against the panel we could identify the specificity of three sera. Two sera were directed to the neutrophil-specific antigen NA2, and the third one reacted with a hitherto not yet recognized neutrophil-specific alloantigen which we called NE1. In 4 patients we could confirm the autoimmune character of the disease by demonstrating the antibody on the patients’ own granulocytes. These results suggest that autoimmunity may be the cause of many cases of benign infantile neutropenia.