In the present study, a murine monoclonal antibody, RK-4 (IgG1) specific for rat granulocytes and their precursors in blood and bone marrow is described. Results obtained by immunocytological and flowcytometry analysis showed this mAb to recognize selectively a surface antigen on neutrophils, eosinophils and basophils. Analysis of cryosections of various lymphatic and non-lymphatic organs demonstrates that this mouse antibody does not cross-react with any other structures. By flow cytometric analysis we found peripheral blood leukocytes to consist of about 15-20% granulocytes. RK-4 is therefore a valuable tool for studying differentiation and function of granulocytes under various conditions.