The B cell-specific mb-1 gene was recently reported to encode a putative surface glycoprotein with CD3-like structural properties. Hombach et al. suggested and presented evidence to show that this mb-1 gene encodes the 34-kDa membrane glycoprotein (B34 or IgM-alpha) associated with IgMR molecule. To identify the mb-1 gene product directly in B cells, affinity-purified MB-1-specific antibody was prepared by immunization of rabbits with synthetic MB-1 oligopeptide. Immunoprecipitation in combination with two-dimensional diagonal gel electrophoresis analysis revealed that this antibody detected a B cell-specific surface glycoprotein that is very similar to the IgM-alpha (B34) protein described by Hombach et al. However, MB-1 protein exists usually as the monomeric form on the surface of B cells, in contrast to IgM-alpha, which was detected as the dimeric (IgM-alpha/IgM-alpha or IgM-alpha/Ig-beta) protein. We also found that MB-1 protein is already expressed on the sIgM- pre-B cell lymphoma, which might suggest an alternative functional role of this B cell-specific MB-1 protein in B cell differentiation. The molecular identity of MB-1 protein and IgM-alpha (B34) is discussed.