The T-cell-specific cell-surface receptors CD28 and CTLA-4 are important regulators of the immune system. CD28 potently enhances those T-cell functions that are essential for an effective antigen-specific immune response^,,,,, and the homologous CTLA-4 counterbalances the CD28-mediated signals and thus prevents an otherwise fatal overstimulation of the lymphoid system^,,,. Here we report the identification of a third member of this family of molecules, inducible co-stimulator (ICOS), which is a homodimeric protein of relative molecular mass 55,000–60,000 (M_r 55K–60K). Matching CD28 in potency, ICOS enhances all basic T-cell responses to a foreign antigen, namely proliferation, secretion of lymphokines, upregulation of molecules that mediate cell–cell interaction, and effective help for antibody secretion by B cells. Unlike the constitutively expressed CD28, ICOS has to be de novo induced on the T-cell surface, does not upregulate the production of interleukin-2, but superinduces the synthesis of interleukin-10, a B-cell-differentiation factor. In vivo, ICOS is highly expressed on tonsillar T cells, which are closely associated with B cells in the apical light zone of germinal centres, the site of terminal B-cell maturation. Our results indicate that ICOS is another major regulator of the adaptive immune system.