We wished to determine whether CD4+ T cells could reject a skin graft that was discordant for a single minor transplantation Ag in the absence of CD8+ T cells or Ab. Transgenic A1 (M) mice were constructed that express the rearranged Vβ8. 2 and Vα10 TCR genes from a T cell clone that is specific for the male Ag (HY) in the context of H2-E k. In addition, the RAG-1−/− background was bred onto these mice to eliminate any endogenous TCR rearrangements. As expected, clonal deletion was found to be complete in the thymus of male A1 (M)× RAG-1−/− mice, while only CD4+ T cells were positively selected and found in the periphery of females. Female A1 (M)× RAG-1−/− mice were able to rapidly reject (in< 14 days) male (but not female) skin grafts in a CD4-dependent fashion. After multiple grafts, it was confirmed that no CD8+ T cells or surface Ig+ B cells were present. An immunofluorescent analysis of spleen cells after grafting showed that the majority of T cells expressed activation markers (CD44, CD25, and intracytoplasmic IL-2) and a significant proportion were making IFN-γ and IL-4. Surprisingly, the transfer of either Th1 or Th2 CD4+ T cell lines from these mice into T cell-depleted recipients was sufficient to cause a specific rejection of male skin.