Abstract We recently found that human CD83, a marker of mature dendritic cells, is an adhesion receptor that binds to resting monocytes and a subset of activated CD8+ T cells. We injected CD83-Ig into mice transplanted with the immunogenic P815 mastocytoma and showed that it significantly enhanced the rate of tumor growth and inhibited the development of cytotoxic T cells. In contrast, mice immunized with CD83-transfected K1735 cells, a poorly immunogenic melanoma, could prevent the outgrowth of wild-type K1735 cells. Studies performed in vitro with human PBL showed that coimmobilized CD83-Ig and anti-CD3 enhanced T cell proliferation and increased the proportion of CD8+ T cells. CD83-transfected B-lymphoblastoid T51 cells stimulated T cell proliferation more effectively than untransfected T51 cells in MLR cultures and increased the generation of cytolytic T cells. We conclude that CD83 is a functionally important receptor that can regulate the development of cellular immunity by interacting with its ligand(s).