To analyze the functions of T cell costimulators in vivo, we have constructed a transgenic mouse strain that constitutively expresses murine B7-1 on mature B cells. Antibody responses to T-dependent hapten-protein conjugates and serum immunoglobulin levels are markedly depressed in B7-1 transgenic mice. This immune deficiency is not due to an intrinsic B cell defect, as antibody responses to T-independent hapten conjugates are normal. Furthermore, treatment with anti-B7-1 restores the capacity of transgenic mice to respond to hapten-protein conjugates, demonstrating that the deficient antibody responses are directly attributable to the expression of B7-1. These results suggest that the temporally regulated expression of costimulators such as B7-1 may contribute to either initiation or down-regulation (feedback inhibition) of T-dependent immune responses in vivo, and that the inhibitory function is dominant in transgenic mice that constitutively express high levels of this costimulator.