A number of the glycoproteins identified on the surfaces of cells of the immune response belong to the immunoglobulin superfamily. We have isolated and characterized cDNA clones and the complete genomic gene encoding a B-cell-specific member of the immunoglobulin superfamily called "B29." This isolate is expressed at all stages in B-cell development beginning with the earliest precursor B cells undergoing immunoglobulin heavy chain gene diversity region----joining region gene (DH----JH) rearrangements. The protein sequence predicted by the B29 coding region contains a leader sequence and a single extracellular immunoglobulin-like domain, followed by a hydrophobic transmembrane segment and a charged intracytoplasmic domain. The immunoglobulin-like domain contains cysteines and other conserved amino acids characteristic of light chain variable and joining regions, but overall the sequence is only distantly related to immunoglobulins. Each of these domains is encoded in separate exons in the B29 gene, in analogy to other members of the immunoglobulin superfamily. The conserved structural features of the immunoglobulin-like domain in the B29 gene product resemble those of other members of the immunoglobulin superfamily involved in cell recognition and adhesion.