One hour incubation with the thymic extract TP-1 induced reciprocal effect on B and T rosette formation in lymphocytes of human peripheral blood. The percentage of mouse erythrocyte rosette-forming cells among lymphocytes of chronic lymphatic leukaemia was decreased by TP-1 from 54.5% to 27.1% (P < 0.001). No such effect was observed in healthy adult or cord blood lymphocytes. On the other hand, the percentage of sheep erythrocyte rosette forming cells increased significantly after TP-1 treatment, but only under conditions of active rosette formation and not in the total rosette assay. This increase was highly significant in three conditions with relative deficiency of cell-mediated immunity: newborns (17.1 to 28.3%), cancer patients (24.5 to 31.7%) and patients with lepromatous leprosy (19.8 to 31.8%). Only a small increase was noticed in healthy adults. A similarly prepared spleen extract was not active in either B or T rosette assays.