SUMMARY Investigations done in rats sensitized to allogeneic tissue antigens have shown that determination of the rate of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) synthesis of leukocytes from blood and spleen provides a rapid method for early detection and measurement of the cellular immune response associated with the induction of transplantation immunity. Lymphoblastic transformation, as reflected in elevated rates of radiolabeled nucleoside incorporation, was detectable 2 days postsensitization and continued to rise on the subsequent days in a pattern paralleling the intensity of the process of skin allograft rejection, as manifested histologically by progressive round cell infiltration. Humoral antibodies became detectable when the rejection of skin grafts was almost completed, and their appearance coincides with the decline of blast cell counts. The evidence in this paper suggests that the elevated rates of DNA and RNA synthesis in peripheral blood mononuclear cells during allograft rejection reflects the appearance of immmunoblasts in the circulation.