Mice have been treated in vivo with xenogeneic antiidiotypes prepared against a murine monoclonal anti-H-2Kk antibody, 11-4.1. B cell immune responses have been found to be altered by such treatment as evidenced by a modification in the idiotypic repertoire of the humoral response to H-2 antigens. Transfer of purified T cells into nude mice before anti-idiotype treatment showed that T cells are involved in the induction of idiotope-bearing antibodies by xenogeneic antiidiotype. Studies using bone marrow chimeras indicate that the environment in which either T or B cells mature does not appear to alter VH region genetic control of induction of antiidiotype-induced molecules. By adoptive transfer studies, T cells from antiidiotype-treated mice were found capable of modifying the idiotypic repertoire of B cells subsequently exposed to antigen even when the T cells were obtained from antiidiotype-primed mice of inappropriate allotype. Although it still must be determined whether idiotypic or antiidiotypic T cells are involved in such B cell idiotype regulation, these results indicate that some T cell functions are altered by xenogeneic antiidiotypes prepared against B cell products and suggest that T cell immunity to major histocompatibility complex antigens may also be affected by such reagents.