EO-1, an IgGl murine monoclonal antibody raised against human eosinophilic leukemia cells, reacts with eosinophils, basophils, platelets, and a few (2%) mononuclear cells but not with neutrophils. In the bone marrow, mature and immature eosinophils and basophils express the EO-1 antigen, whereas immature myeloid cells do not. The distribution of EO-1 antigen on leukemic cells is concordant with this finding, ie, typical myeloid lines (HL-60, KG-1, and ML-1) and fresh acute myelogenous leukemia cells are all unreactive with EO-1. Immunoprecipitation of an extract from surface-131I-labeled platelets with EO-1 and sodium dodecyl sulfate polyacrylamide gel electrophoresis, under reducing or nonreducing conditions, yielded a specific band of molecular weight 23,000. Previously described monoclonal antibodies reacting with eosinophils also recognize neutrophils. EO-1 is a unique antibody with specificity restricted to eosinophils, basophils, and platelets and might therefore be a valuable reagent for the study of their function and differentiation.