The mb-1 gene encodes a protein associated with membrane-bound immunoglobulins (mIg) and it has been suggested that it may play a role in signal transduction. Using a murine probe, we cloned a human complementary DNA homologous to the murine mb-1 sequence. Its complete sequence is in full agreement with a recently published human cDNA sequence but differs from a previously reported partial sequence. We studied mb-1 expression in human cells at various maturation stages. Large amounts of a single 1.4 kb transcript were detectable in B cell lines carrying mIg of the mu, gamma, alpha 1 or alpha 2 isotype. The mb-1 mRNA was also expressed in pre-B cells and in cells expressing truncated membrane mu chains devoid of associated light chains. Only trace amounts of mb-1 mRNA were found in tissue samples containing numerous plasma cells, while no mRNA was detected in several plasmacytoma cell lines, indicating that expression is shut down in plasma cells. No signal was found in T cells or in non-lymphoid tissues. Altogether, these results show that the human mb-1 gene is expressed in pre-B cells and B lymphocytes, regardless of the isotype of the mIg heavy chain and is no longer expressed in plasma cells.