Cellular components of the developing fetal nephron express certain cell surface antigens at discrete stages of nephron development. To further evaluate the expression of antigens by tubular epithelium, a rat monoclonal antibody (BL26) was developed against murine fetal kidney. BL26 identified a differentiation antigen expressed by epithelial cells of the ureteric bud and late S-body in developing nephrons and by endothelium, mesangium, and distal tubular epithelium in mature nephrons. The polypeptide identified by BL26 was shown to be synthesized by fetal kidney and renal adenocarcinoma cells. The polypeptide contained no detectable carbohydrate modifications but was found in two isoforms, an acylated (26,000-Da) form and a nonacylated (24,000-Da) form. The murine antigen recognized by BL26 and the human CD9 moiety, an acylated polypeptide with a tissue distribution similar to that of the BL26 antigen, comigrated in polyacrylamide gels. We speculate that expression of the BL26 antigen and CD9 reflects processes relating to the activation of epithelial cells.