To provide further support of the B-cell lineage of "common" acute lymphoblastic leukemia (ALL), we studied the REH cell line and found not only cytoplasmic heavy chain but also surface lambda light-chain expression by flow cytometry. Nucleic acid blotting experiments on REH revealed that both kappa genes had been deleted and that lambda genes had been rearranged, as expected in B cells expressing lambda light chain. Moreover, REH cells contained mu and lambda RNA. When REH cells were treated with TPA the amount of mu chain RNA increased by approximately fivefold and the amount of lambda chain RNA increased by approximately twofold. The finding of surface immunoglobulin lambda light chain in the REH cell line, suggests that these leukemic cells are further differentiated along the B-cell lineage than was previously believed.