A monoclonal antibody (mAb) G‐5‐2 was isolated which binds to transformed as well as normal cells of the B lineage but not to cells of the T cell, myeloid lineages nor to fibroblasts. mAb G‐5‐2 reacts with pre‐B and plasma cell‐transformed lines, and it preferentially recognizes normal pre‐B cells from fetal liver and bone marrow as well as plasma cells from spleen of mice. G‐5‐2⁺ fetal liver cells isolated by cell sorter express mRNA for μ heavy chain Ig gene and generate in vitro antibody‐producing cells when co‐cultured with lipopolysaccharide and rat thymocyte filler cells. During development the frequency and staining intensity of G‐5‐2⁺ cells in fetal liver from normal mice increases from 1% G‐5‐2⁺ cells at day 14 to∼7% positive cells at day 18 of gestation. Several strains or normal mice contain comparable numbers of G‐5‐2⁺ cells as well as B‐220⁺ and BP‐1⁺ B cell precursors in the fetal liver. Mice carrying the xid mutation have 3‐4‐fold less G‐5‐2⁺ as well as B‐220⁺ and BP‐1⁺ cells in the fetal liver, suggesting that the effects of the xid mutation may be manifested from early stages of B cell development. Fetal liver cells from mice carrying the scid mutation were found to contain normal numbers of G‐5‐2⁺ as well as B‐220⁺ and BP‐1⁺ pre‐B cells. These results indicate that differentiation from progenitors to pre‐B cells in scid mice may occur normally; the scid mutation would thus appear to affect the process of rearrangement and expression of the Ig genes in the developing pre‐B cells. mAb G‐5‐2 precipitates a 76‐kDa glycoprotein from surface‐radiolabeled pre‐B cells and plasma cells. Taken together, these results indicate that G‐5‐2 mAb recognizes a novel B cell lineage‐specific surface molecule called PB76 which is preferentially expressed by pre‐B cells and plasma cells.