A number of tumor studies have indicated a link between CD4 help and the magnitude and persistence of CTL activity; however, the mechanisms underlying this have been largely unclear. To evaluate and determine the mechanisms by which CD4+ T cells synergize with CD8+ T cells to prevent tumor growth, we used the novel technique of monitoring in vivo CTL by labeling target cells with CFSE. This approach was supported by the direct visualization of CTL using peptide-MHC tetramers to follow tumor-specific T cells. The data presented demonstrate that while cotransfer of Ag-specific CD4+ T cells was not required for the generation of CTLs, because adoptive transfer of CD8+ T cells alone was sufficient, CD4+ T cells were required for the maintenance of CD8+ T cell numbers. Our data suggest that there is a correlation among the number of CD8+ T cells, in vivo CTL function, and IFN-γ production, with no evidence of a partial or nonresponsive phenotype among tetramer-positive cells. We also show that CD4+ T cells are required for CD8+ T cell infiltration of the tumor.