The biological function of 2B4, a CD48-binding molecule expressed on T cells with an activation/memory phenotype, is not clear. In this report, we demonstrate that proliferation of CD8+ T cells is regulated by 2B4. Proliferative responses of CD8+ T cells were significantly reduced by anti-2B4 Ab. The effects were not potentiated by anti-CD48 Ab, suggesting that the observed responses were driven by 2B4/CD48 interactions. Surprisingly, the 2B4/CD48-dependent proliferative responses were also observed in the absence of APCs. This suggests that 2B4/CD48 interactions can occur directly between T cells. Furthermore, when activated 2B4+ CD8+ T cells were mixed with 2B4− CD8+ TCR-transgenic T cells and specific peptide-loaded APC, the proliferation of the latter T cells was inhibited by anti-2B4 Ab. Taken together, this suggests that 2B4 on activated/memory T cells serves as a ligand for CD48, and by its ability to interact with CD48 provides costimulatory-like function for neighboring T cells.