The acute stage of feline immunodeficiency virus (FIV) infection is characterized by a CD8+ anti-FIV response that parallels the appearance of a CD8+ subpopulation with reduced expression of the β chain (CD8α+βlo). The relationship between the CD8α+βlo phenotype and CD8+ anti-FIV activity was examined. Flow cytometric analysis of peripheral blood mononuclear cells with anti-CD8 β chain monoclonal antibody 117 revealed that the CD8α+βlo phenotype expanded throughout the asymptomatic infection, constituting 80%-90% of the CD8β+ cells in long-term-infected cats. Purified CD8α+βhi and CD8α+βlo subpopulations were analyzed for anti-FIV activity in an acute infection assay. Anti-FIV activity resided principally in the CD8α+βlo population and was demonstrated in acute FIV infections, as well as in long-term asymptomatic infections. These data suggest that a unique CD8α+βlo anti-FIV phenotype arises early in infection and may play a major role in eliminating virus and maintaining the asymptomatic infection.