We describe successful immunotherapy of murine AIDS (MAIDS) in C57BL/6J mice based on the elimination of replicating CD4+ regulator T cells. We demonstrate that a single injection of the antimitotic drug vinblastine (Vb) given 14 days postinfection (pi) with LP-BM5 can prevent MAIDS progression. Treatment with anti-CD4 mAb at 14 days pi is similarly able to prevent MAIDS. Treatment at other time points with Vb or anti-CD4 mAb is ineffective. The effect is based on ablation of a replicating dominantly suppressive CD4+ T cell population, as indicated by adoptive transfer and in vivo depletion experiments using mAbs against CD4 as well as combinations of mAbs against the known regulatory cell surface markers CD25, GITR, and CTLA-4. Cell surface marker analysis shows a population of CD4+ CD25+ cells arising shortly before day 14 pi Cytokine analyses show a peak in IL-10 production from day 12 to day 16 pi MAIDS-infected mice also have CD4+ T cells with significantly higher expression levels of CD38 and particularly CD69, which have been demonstrated to be regulator T cell markers in the Friend retroviral model. The immunotherapy appears to prevent disease progression, although no protection against reinfection with LP-BM5 is generated. These data define a new therapy for murine retroviral infection, which has potential for use in other diseases where T regulator cell-mediated immunosuppression plays a role in the disease process.