It is known from several experimental systems that the production of dominant antibody idiotypes may be regulated by anti-idiotypic mechanisms. Our aim has been to test whether the potential for such control also exists in the more typical heterogeneous antibody responses of inbred mice to the trinitrophenyl (TNP) hapten, where dominant idiotypes are not recognized. CBA mice were hyperimmunized to trinitrophenylated keyhole limpet hemocyanin. Serum or lightly fixed spleen cells from these mice were injected into normal syngeneic "recipients". The serum and spleen cells from these recipients were found to have the power to suppress in vitro anti-TNP antibody responses made by further spleen cells from the donor mice. This suppression was specifically directed against the cells of the individual donor animals suggesting idiotype-related regulation.