Cytotoxicity of lymphocytes, mixed lymphocyte cultures, stimulation with PHA, suppressive activity of cells in vitro and efficacy of the adoptive transfer of cells in vivo were simultaneously investigated in rats bearing tolerated skin allografts. Cells from animals which did not react in MLC were simultaneously hyporeactive when stimulated with PHA. On the other hand, cytotoxicity of lymphocytes did not correlate with their ability to respond to specific antigens in MLC. None of the in vitro tests correlated with the efficacy of the adoptively transferred syngeneic lymphoid cells.