The T cell populations that respond to autologous or allogeneic non-T cells are distinct, at least to a considerable extent. This conclusion is based on the differences in a) the size of the two populations; on the average one in 260 (individual frequency range, 1:149 to 1:417) T cells respond to allogeneic non-T cells whereas one in 3100 (individual frequency range, 1:2200 to 1:5000) T cells respond to autologous non-T cells; b) the phenotype of T cells activated in the MLR; autoactivated T cells were drawn predominantly from the OKT4-reactive T cell subpopulation, whereas alloactivated T cells activated in the allogeneic MLR were drawn from the OKT4, OKT5, and OKT8-reactive T cell subpopulations; and c) the specificity of activated and nonactivated T cells obtained on Percoll gradients after autologous or allogeneic MLR. Auto- or allodepleted T cells recovered from the 55 to 60% Percoll interface retained full alloreactivity or autoreactivity, respectively. However, auto- or alloactivated T cells recovered from the 40 to 50% Percoll interface showed secondary kinetics to allogeneic and autologous non-T cells. Whether the alloreactivity or autoreactivity found in autoactivated or alloactivated T cells, respectively, represents true cross-reacting T cells or T cells nonspecifically recruited during the MLR cannot at present be decided.