We prospectively evaluated the usefulness of measuring antineutrophil (PMN) antibodies in patients with neutropenia by a sensitive immunoassay. The result of the immunoassay were compared to a standard leukoagglutination test. Thirty-two patients with neutropenia were studied. The mean value ( ± 1 SD) for PMN-bound IgG for 43 normal controls was 121 ± 29 fg/PHN. All fifteen patients with immune neutropenia had elevated plasma levels of anti-PMN antibody (191–2,000 fg/PMN). The leukoagglutination assay was positive in 10 of these 15 patients. In 17 patients with nonimmune neutropenia, the mean value ( ± 1 SD) for PMN-bound IgG was 120 ± 22 fg. Seventeen patients with sero-positive rheumatoid arthritis but normal PMN counts were also studied. These patients’ plasmas bound 447 ± (1 SD) 201 fg IgG/PMN. The leukoagglutination test was negative in these patients. Our study shows that the immunoassay is more sensitive than leukoagglutination testing for diagnosing immune neutropenia. Nonimmune, neutropenic patients’ plasmas showed no increased reactivity. In nonneutropenic patients who have high titers of rheumatoid factor, immunoreactivity against PMN is measured by the immunoassay, but not by leukoagglutination testing. Binding assays may be more sensitive than leukoagglutination testing for measuring anti-PMN antibodies; however, in some cases, specificity of such testing may be increased with leukoagglutination testing.