We have recently reported the isolation of cDNA clones encoding the human homolog of the mouse B cell differentiation antigen Lyb-2. Expression of Lyb-2 is restricted to B lineage cells and turned off in antibody-secreting plasma cells in both mice and humans. Functional studies with anti-mouse Lyb-2 monoclonal antibodies (mAb) suggest that this protein may be involved in signals for B cell proliferation. We now describe the generation of mAb specific for human Lyb-2. Furthermore, we demonstrate that the human Lyb-2 protein is recognized by a mAb specific for the newly clustered pan B cell surface antigen CD72, which had been defined by mAb. Mouse L(tk-) cells expressing a transfected cDNA encoding human Lyb-2 bind a mAb specific for CD72. We further show that an antiserum and a mAb specific for human Lyb-2 and an anti-CD72 mAb immunoprecipitate the identical protein from human splenic B cells and B cell lines, as well as from transfected L(tk-) cells. These data indicate that CD72 is the human equivalent of mouse Lyb-2. We have additionally localized the gene for human Lyb-2/CD72 on the short arm of chromosome 9. Mouse Lyb-2 had been previously mapped to mouse chromosome 4. Lyb-2/CD72 is now the tenth gene known to be localized on human chromosome 9 and mouse chromosome 4.