The influence of the immunologic status of the cell donors on the proliferative behavior of rat lymphocytes in the mixed lymphocyte interaction has been studied. Mixed cultures of cells from various parental and F 1 combinations having morphologically distinguishable sex chromosomes exhibited unidirectional proliferative reactivity. The mitotic figures were predominately of parental origin. Lymphocytes from donors made tolerant at birth to homologous transplantation isoantigens were specifically unreactive against cells bearing antigens of the tolerance inducing strain, but not to indifferent third party homologous lymphocytes. Cells from animals that had been surgically thymectomized at birth exhibited a markedly and sometimes totally diminished reactivity against homologous lymphocytes. Presensitization of the cell donors resulted in a curtailment of proliferative reactivity in cultures with cells bearing the immunizing antigens. This may reflect the destructive properties that lymphocytes from sensitized animals are known to possess. The results of these experiments show that the proliferative activity of lymphocytes in the mixed lymphocyte interaction accurately reflects the immunologic status of the cell donors, and these findings provide further support for the premise that the mixed lymphocyte interaction represents a primary immunologic response by cells in culture against homologous cells bearing histocompatibility antigens.