Mice that are suppressed with respect to an idiotype (CRIA) present in A/J anti-p-azophenylarsonate antibodies, hyperimmunized, and allowed to rest were previously found to possess high concentrations of suppressor T cells with anti-idiotypic receptors. We have now observed that the sera of such mice contain soluble factors that can selectively suppress the CRIA component of a humoral response when passively transferred to adult or neonatal recipients. When T cells from suppressed, hyperimmunized mice were transferred into female mice before mating, their offspring, upon immunization, produced anti-Ar antibodies that lacked CRIA. A state of idiotypic suppression was also produced in offspring when the mother was inoculated with serum from suppressed mice a few days before parturition. The results indicate that the suppressor factor is not an immunoglobulin.