Superantigens are defined by their ability to stimulate T cells based predominantly on their Vβ expression and ability to delete T ceils in the thymus when expressed endogenously. We show here that the expression of one endogenous superantigen, Etc-1, is controlled by the expression of the open reading frame region of the 3′ long terminal repeat of the mouse mammary tumor proviral gene, Mtv-9. We show that Mtv-8 controls a superantigen with similar specificity, and that both Mtv-8 and Mtv-9 stimulate some Vβ17+ T cells. A third provirus, Mtv-6, controls a superantigen with specificity for Vβ3. Data presented raise the possibility that endogenous superantigens may compete for class II molecules in a single B cell. © 1991, Rockefeller University Press., All rights reserved.