Adhesion of T lymphocytes is an essential step for antigen recognition and lymphocyte activation. mAbs to T cell surface proteins have been used to define the receptor-ligand proteins that appear to be involved in adhesion. Since most assays measure the effects of mAbs on T lymphocyte function, it is not known whether mAb-mediated blocking is due to a disruption of receptor-ligand interactions or results in inhibition of some aspect of receptor-mediated triggering. It has been suggested that the CD8 molecule augments T cell avidity for the target cells by binding to determinants on target cell MHC class I molecules. In the present report, we demonstrated that purified CD8 molecules incorporated into large lipid vesicles (artificial target cells) mediate the adhesion of these vesicles to cells expressing HLA proteins, while vesicles expressing purified HLA class I antigens bind to CD8+ T cells. Furthermore, vesicles bearing CD8 will form conjugates with vesicles expressing HLA class I proteins. These conjugates were found to be specifically inhibited by mAbs to CD8 or HLA class I molecules. We also demonstrate that CD2-reconstituted vesicles can form conjugates with vesicles bearing LFA-3. These experiments provide direct evidence for an interaction of the CD8 molecule with class I MHC proteins as well as between CD2 proteins and LFA-3 proteins, thus supporting the hypothesis that these molecules can mediate cell-cell adhesion.