A number of milk-borne exogenous mammary tumor viruses (MMTV) infect mice shortly after birth and, when expressed, produce superantigens. The expression of these superantigens mediate the progressive deletion of T cells expressing specific V beta products. Here we describe a maternally-inherited alteration in the T cell repertoire in one colony of BALB/c mice which has not been reported up to now. This alteration involves the deletion of V beta 2+ and 14+ CD4+ T cells and correlates with a high incidence of mammary tumors, suggesting the involvement of a superantigen(s) probably transmitted through an exogenous MMTV in milk.