Sera from 15 patients with antineutrophil antibodies against peripheral blood neutrophils were incubated with normal bone marrow, and the reaction with marrow precursors was determined. Serum from controls did not deposit IgG on either peripheral blood or marrow myeloid cells. Serum from patients with moderate immune neutropenia with myeloid hyperplasia deposited IgG on neutrophils, bands and metamyelocytes, but not on earlier precursors. Serum from one patient with severe neutropenia and myeloid hypoplasia deposited IgG on myelocytes and promyelocytes as well as more mature cells. Serum from a patient with antineutrophil antibodies which impaired chemotaxis but who did not have neutropenia deposited IgG only on neutrophils and occasional bands. Immune neutropenia may become increasingly severe as antibodies become directed against earlier precursors.