T cell interaction with endogenous retroviral superantigens in vivo results in either deletion of the reactive T cells or their transition to a state of unresponsiveness. We show here that introducing cells expressing the endogenous superantigen Mls 1a into an Mls 1b environment results in the induction of an anergic state with little or no depletion of V beta 6/CD4+ T cell from the peripheral T cell repertoire. Removal of B cells from the priming innoculum abrogates the induction of tolerance, indicating that presentation of Mls 1a by B cells induced the tolerance observed. The tolerant T cells are unable to respond to either presentation of Mls 1a on spleen cells or cross-linking anti-T cell receptor antibody in vitro. Although large increases in the cell size and numbers of reactive T cells are observed 2-3 days after priming with Mls 1a-presenting cells, this activation is not a requirement for the induction of tolerance because irradiated Mls 1a-presenting cells retained the ability to induce tolerance without these changes in the reactive T cell population. Furthermore, providing host T cells with Mls 1a and a costimulatory signal, in the form of high levels of the B7 determinant on the donor cells, did not circumvent the induction of tolerance. Collectively, these results indicate that the transition to an anergic state following interaction with endogenous superantigen in vivo is not dependent upon the activation of the reactive T cells.