CD4(+)CD8(dim) T cells represent a minor subset of the total CD3(+) T cell population in peripheral blood. Although transient and persistent expansions of these cells have been reported in both healthy and diseased individuals, the functional properties of the CD4(+)CD8(dim) population are largely unknown. In this study, we examined antigen-specific cytokine and proliferative responses of the CD4(+)CD8(dim) subset. In whole blood cultures stimulated with the viral antigens HCMV and HIV-1, a significant fraction of the CD4(+)CD8(dim) subset exhibited cytokine expression and proliferation in response to antigen activation. Typically, the CD4(+)CD8(dim) population contained two- to eightfold higher frequencies of antigen-specific cytokine producing cells than the CD4(+)CD8(-) population. Phenotypic analysis of the cytokine expressing CD4(+)CD8(dim) population indicated that these cells are memory T cells, with a high frequency of this population expressing the cytotoxic markers CD56 and perforin. Furthermore, the CD4(+)CD8(dim) cytokine responses to CMV were shown to be MHC class II dependent. Significantly, purified CD4(+)CD8(dim) T cells were found to possess higher CMV-specific cytotoxic activity than purified CD4(+)CD8(-) T cells in a standard (51)Cr-release CTL assay. Thus, CD4(+)CD8(dim) T cells appear to be MHC class II dependent, are capable of cytolytic effector activity, and are highly enriched within the CD4(+) cell populations specific for HCMV and HIV-1.