A novel glycoprotein of 70kDa, designated SLAM (Signaling Lymphocyte Activation Molecule) / CDw150, that belongs to the immunoglobulin gene superfamily and is involved in T-cell stimulation. SLAM is constitutively expressed on peripheral blood memory T cells, T-cell clones, immature thymocytes, and a proportion of B cells, and is rapidly induced on naive T cells after activation. Activated B cells express the membrane-bound form of SLAM and the soluble and cytoplasmic isoforms of SLAM, and that the expression levels of membrane-bound SLAM on B cells are rapidly regulated after activation in vitro. It is suggested that signaling through homophilic SLAM-SLAM binding during B-B and B-T cell interactions enhances the expansion and differentiation of activated B cells.