A gene, called m‐mb‐1, was isolated from a murine pre‐B‐minus T lymphocyte subtracted library. It was found expressed as mRNA at low to medium abundance in early progenitors of the B lineage, in pre‐B and mature B lineage cell lines, in normal resting B lymphocytes and in polyclonally activated B cell blasts. The gene was not expressed in plasmacytomas, in cell lines of the monocyte/macrophage, the T lymphocyte or the fibroblast lineages, nor in thymus, liver, heart, kidney, lung or brain. The nucleotide sequence of the m‐mb‐1 gene encodes a putative membrane glycoprotein with 220 amino acids, which includes a leader sequence, a putative extracellular domain with two potential N‐glycosylation sites, a transmembrane portion and a putative intracellular domain. The partial sequence of a human homologue, h‐mb‐1, shows nearly 90% homology in nucleotide as well as amino acid sequences to the murine form of a stretch of the putative intracytoplasmic domain. Antibodies raised against a fusion protein of m‐mb‐1 with protein A, affinity purified for their m‐mb‐1 specificity, stained pre‐B and mature B cell lines on their surface, but did not stain T cell lines and fibroblasts. Antibodies raised against a stretch of 20 amino acids of the putative intracellular domain with 90% homology between the mouse and human protein did not stain the surface of any cell lines tested.(ABSTRACT TRUNCATED AT 250 WORDS)