AbstractThe murine monoclonal antibody (mAb) CB43, raised against the K‐562 erythroleukemia line, reacts with monocytes, tissue macrophages, thymocytes and with all the human lines tested but not with resting lymphocytes, large granular lymphocytes, granulocytes and erythrocytes. However, activated lymphocytes and natural killer cells express the CB43 antigen. Embryonic and fetal fibroblasts are positive, while adult fibroblasts are negative. Proximal convoluted tubules in kidney, epithelial cells in esophagus and breast, and sarcolemma in skeletal muscle are reactive with mAb CB43. This antibody can also bind to isolated guinea pig cardiac myocytes, and, furthermore, can induce a transient inotropic effect on isolated atria.The reactivity with different cell and tissue types and the functional effects of the CB43 mAb were reminescent of the 4F2/44D7 antibodies, shown previously to block Na/Ca2+exchange in heart and skeletal muscle. Co‐immunoprecipitation studies with CB43 and 44D7 mAb, using radiolabeled Daudi cells, revealed co‐migration of polypeptides of 87 and 38 kDa.However, the epitope recognized by CB43 is not present on the human heavy chain which bears the 44D7/4F2 epitope as demonstrated by the lack of reactivity of CB43 mAb with mouse L cells transfected with the 4F2 heavy chain gene. Thus, CB43 represents a newly described epitope present on the human light chain or dependent on the conformation of the human