Expression of CD2 on developing and mature murine B cells was examined by using an antipeptide antiserum (L50). Most Ig-bearing splenic B cells were found to express CD2. Anti-CD5 and anti-B220 mAb divided the peritoneal B cells into two populations expressing high and low levels of these proteins; both populations were found to express uniform levels of CD2. Abelson murine leukemia virus-transformed pre-B cell lines derived from fetal liver and adult bone marrow were analyzed to delineate the ontogeny of CD2 in the B cell lineage. The results show that onset of CD2 expression correlates with the presence of cytoplasmic mu-chain. Therefore, the earliest CD2+ pre-B cell in the developing B cell population appears to be the classical pre-B cell.