The interaction of B7-related molecules on antigen-presenting cells with CD28 or CTLA-4 antigens on T cells provides a second signal for T cell activation. Selective inhibition of the B7-CD28 or B7-CTLA-4 interactions produces antigen-specific T cell unresponsiveness in vitro and suppresses immune function in vivo. To determine whether selective inhibition of the B7-CD28 or B7-CTLA-4 interactions could suppress spontaneous autoimmune disease, a B7-binding protein was generated by genetic fusion of the extracellular domain of murine CTLA-4 to the Fe portion of a mouse immunoglobulin G2a monoclonal antibody (muCTLA4lg). In lupus-prone NZB/NZW filial generation (F-1) mice, treatment with muCTLA4lg blocked autoantibody production and prolonged life, even when treatment was delayed until the most advanced stage of clinical illness. These findings suggest a possible role for human CTLA4lg in the treatment of autoimmune diseases in humans.