Six antineutrophil antibody (ANA)-positive patients with copper deficiency were classified into two groups; those with (group A, n =3) and those without (group B, n = 3) neutropenia. The percent binding of ANA for normal peripheral neutrophils was similar in both groups (8 3.5 ± 7.2 vs. 79.1 ± 10.5%). The percent binding of sera to cultured promyelocytic leukemia cells (HL-60) was increased in group A (from 3.7 ± 3.2 to 12.2 ± 2.3%) but not in group B (from 65.3 ± 21.7 to 40.7 ± 6.3%) after stimulation of HL-60 with DMSO. The stimulated HL-60 cells expressed CD 16 and CD 11b antigens. In the presence of monoclonal antibody for CD 16, the titer of ANA was nil in group A and unchanged in group B. Thus, ANA of patients with neutropenia may recognize mainly the CD 16 antigen, the Fcγ receptor III of neutrophils.