SummaryAn antigen expressed by most human cells, but not erythrocytes, has been defined by a murine monoclonal antibody, TRA‐2–10. This antigen is expressed on the surface of human‐mouse somatic cell hybrids, and segregation analysis indicates that it is controlled by a gene located on human chromosome 1. From lysates of most human cells, surface‐labelled with125I, TRA‐2–10 immunoprecipitates two polypeptides with molecular weights in the range of about 55000 to 73000 depending upon the cell line. Since the TRA‐2–10 polypeptides from a fibroblast cell strain and a hepatoma cell line from one individual differ, we conclude that the observed difference in molecular weight has an ep