The serum of a 39-year-old male under long-term platelet transfusion therapy for hypoplastic anemia with thrombocytopenia was investigated for the presence of leukocyte and platelet antibodies after the patient had received platelet concentrates from more than 700 random donors. The serological studies of his serum revealed: (1) the absence of plateletreactive antibodies ; (2) the absence of agglutinating or cytotoxic antibodies against leukocytes ; (3) the presence of at least two granulocyte-specific antibodies, one with the specificity anti- NA(2) and the other with an undefined specificity, both only detectable by indirect immunofluorescence, and (4) the presence of cytotoxic-negative fluorescence-positive peripheral-bloodmononuclear- cell-reactive antibodies, not directed against HLA-A, B or C antigens. The significance and implications of these findings are briefly discussed.