The appearance of activated lymphocytes and antibodies in the circulation of rats was monitored following allogeneic kidney, skin, or combined skin-spleen transplants. Lymphocyte activation was measured by 3H-thymidine incorporation, and antibody by a hemolytic assay. These assays have been developed so that small samples of blood could be used, thus maintaining recipient survival and allowing for daily correlation between the response observed and the fate of the graft. These responses were detected after allogeneic transplantation in two recipient strains, but not following isogeneic renal or skin transplantation in either strain. The relative levels, time of onset and persistence of lymphocyte proliferation and of hemolytic antibody were found to differ substantially for the three different kinds of allografts. In general, these studies have demonstrated the validity of using the lymphocyte proliferative and humoral responses together as indicators of a current immune response against allogeneic renal transplants.