Monoclonal antibodies have been obtained that react with the murine leukocyte surface glycoproteins bearing the macrophage differentiation antigen Mac-1. Structural and antigenic analysis shows that related glycoproteins are found on other murine hematopoietic cell types. The data not only illustrate the complexity of cell surface structures that can be detected by means of monoclonal antibodies, but also raise questions as to the functional significance of this family of molecules within the hematopoietic system.