Megakaryoblasts of bone marrow differentiate into megakaryocytes that in turn are the source of blood platelets. We have raised monoclonal antibodies to a megakaryoblast-like cell line derived from rat bone marrow (RPM cells). One antibody (Mab 213) and the corresponding antigen has been characterized by Western blotting and immunohistochemistry. Biosynthetic labeling with [35S]methionine showed that this antigen is synthesized by the RPM cells. In Western blots the antibody recognized proteins of about 90 kDa and 160 kDa in Triton extracts of RPM cells, whereas it recognized proteins of about 160 kDa and 200 kDa in Triton extracts of rat platelets and one of about 200 kDa in Triton extracts of various rat tissues (kidney, lung, intestine, and heart). By immunohistochemistry, the antigen was localized to the apical part of the epithelium lining certain parts of kidney tubuli, bronchi and large intestine.