We describe a simple test for direct or indrect detection of antineutrophil antibodies. Sensitized leukocytes adherent to glass slides and fixed with paraformaldehyde can be stored in buffer for at least 3 wk. Killed Cowan I staphylococci, containing protein A, bind to sensitized but not control cells, and binding is ascertainable by light microscopy. Indirect tests were positive for 39/41 patients suspected of having immune neutropenia and found to have antineutrophil antibodies by an indirect radiochemical opsonic method. Fifty-four control sera from healthy persons, patients with bone marrow failure, or with immune complex diseases without neutropenia, gave negative indirect tests. Direct tests for cell-bound antibody could be done even during severe neutropenia by reacting fixed autologous cells with staphylococci in the absence of added serum. In some patients only the direct test was positive.