The B cell antigen receptor is a complex containing the antigen-binding immunoglobulin molecules and the Ig-alpha/Ig-beta heterodimer which presumably connects the B cell antigen receptor to intracellular signaling components. To analyze the functional properties of the cytoplasmic parts of the B cell antigen receptor, we used the K46 B lymphoma line (IgG2a, kappa) to express chimeric molecules composed of the extracellular and transmembrane part of the CD8 alpha molecule and the cytoplasmic sequence of either the Ig-alpha (CD8 alpha/Ig-alpha), the Ig-beta (CD8 alpha/Ig-beta) protein or the membrane-bound gamma 2a heavy chain (CD8 alpha/gamma 2a). From these three types of chimeric molecules only (CD8 alpha/Ig-alpha and CD8 alpha/Ig-beta, but not CD8 alpha/gamma 2a, could transduce signals, thus providing the first evidence that the cytoplasmic tail of Ig-alpha and Ig-beta have a signaling capacity. After cross-linking with anti-CD8 alpha antibodies, both molecules induced a similar increase in intracellular free calcium ion and in MAP kinase phosphorylation. Protein tyrosine kinases, however, were strongly activated via the CD8 alpha/Ig-alpha and only marginally via the CD8 alpha/Ig-beta molecule. This suggests that the Ig-alpha and Ig-beta proteins have distinct roles during signal transduction through the B cell antigen receptor.