Abstract The expression and function of the novel signaling lymphocytic activation molecule (SLAM) are described. SLAM is present on immature thymocytes, CD45R0(high) memory T cells, T cell clones, CD56+ T cells, EBV-transformed B-cell lines and on a proportion of B cells. SLAM is rapidly induced on naive T cells, TCR gammadelta+ T cells, and B cells following activation. Engagement of SLAM by the agonistic anti-SLAM mAb A12 resulted in proliferation of T cell clones and Ag- or PHA-activated peripheral blood T cells. mAb A12-induced T cell proliferation is independent of IL-2 and sensitive to inhibition by cyclosporin A. The extent of the anti-SLAM-induced proliferation was related to the activation state of the T cells, since fully rested T cell clones failed to proliferate in response to mAb A12 stimulation, despite unchanged SLAM expression on their surface. Ligation of SLAM on activated peripheral T cells or T cell clones by mAb A12 induced IFN-gamma production in the absence of other exogenous stimuli, even in allergen-specific Th2 clones. These data indicate that stimulation via SLAM reverses the cytokine production profile of Th2 clones to a Th0 phenotype, whereas it further polarizes cytokine production by Th1 clones. Thus, SLAM is a novel receptor that mediates IL-2-independent expansion of activated T cells during immune responses, while concomitantly directing these proliferating cells to a Th0/Th1 pathway.