This paper summarizes the results of the characterization of A10, a murine monoclonal antibody which recognizes an epitope not restricted to cells of a definite lineage, but which seems to be specific for an early differentiation antigen present on precursors of circulating T and B cells. The structure recognized by the A10 monoclonal antibody, although strikingly structurally similar to the HLA-A,B,C complex, is immunologically different both from histocompatibility antigens and from beta 2 microglobulin. Furthermore, the distribution of the A10 antigen is analyzed in different cell and tissue samples, both in health and disease conditions.