Mouse mammary tumor virus (MMTV) expresses a superantigen (SAg) which plays a critical role in the viral life cycle. We have recently described the new infectious MMTV (SIM) encoding a Vβ4-specific SAg in mice with a TCR-Vβb haplotype. We have now compared the SAg activity of this virus in BALB / c mice harboring the TCR-Vβa, TCR-Vβb or TCR-Vβc haplotypes which differ by a central deletion in the TCR-Vβa and TCR-Vβc locus and by mutations in some of the remaining Vβ elements. Injection of MMTV (SIM) led to a strong stimulation of Vβ4 + CD4 + T cells in TCR-Vβb mice, but only to a weak stimulation of these cells in TCR-Vβa or TCR-Vβc mice. A large increase in the percentage of Vβ10 + cells was observed among CD4 + T cells in mice with the Vβ a or Vβ c, but not the Vβ b TCR-Vβ haplotype. Vβ 10+ cells dominated the response when Vβ10a/c and Vβ 4 subsets were present together. This is the first report of a viral SAg interacting with murine Vβ10 + cells. Six amino acid differences between Vβ10a / c and Vβ10b could account for the gain of reactivity of Vβ10a / c to the MMTV(SIM) SAg. No mutations were found in the hypervariable region 4 (HV4) of the TCR. Mutations at positions 22 and 28 introduce into Vβ10a / c the same amino acids which are found at these positions in the MMTV(SIM)-reactive Vβ4. Tridimensional models indicated that these amino acids lie close to HV4 and are likely to be important for the interaction of the SAg with the TCR.