Beside the immunoglobulin (Ig) heavy and light chains the murine B cell receptor of the IgM class contains a heterodimer of two transmembrane proteins (IgM-alpha and Ig-beta). By N-terminal sequencing of IgM-alpha and Ig-beta we have identified the genes encoding these proteins as mb-1 and B29, respectively. Both genes are B cell specific and have been previously cloned from B minus T cell subtractive cDNA libraries. We have constructed expression vectors of the two genes and demonstrate that expression of the mb-1 and B29 genes can influence the surface expression of IgM in micron-transfected myeloma cells. From the known sequences of the IgM-alpha and Ig-beta proteins and from the results of previous transfection experiments with various vectors expressing the mu chain we have developed a structural model of the B cell antigen receptor of class IgM which we compare with that of the T cell antigen receptor.