With the aid of the lymphocyte transformation test (LTT) using different T and B cell mitogens it is possible functionally to examine and differentiate various subpopulations of lymphocytes in man. Normal values of clinically healthy persons were evaluated. The T cell type reactive to Phytohaemagglutinin (PHA) has been found most frequently in the blood (mean transformation rate 79%), less frequently in the spleen and very seldom in the thymus. In 3 patients with infectious mononucleosis a B lymphocyte subpopulation stimulated by Lipopolysaccharide (LPS) was demonstrable in their peripheral blood, being nearly absent in normal persons, but clearly evident in the spleen of splenectomized patients and of 57 Bl-mice. Peripheral leucocytes of healthy persons, however, were able to be transformed into either blast cells by anti-isiotypic immunoglobulin sera or plasma-cell-like cells, while chronic lymphocytic leucamia lymphocytes of several patients reacted in a different way--may be, because of a different degree of insufficiency of these cells. For clinical use it should be possible by means of the LTT to evaluate changes in T and B cell transformation caused by disease or by treatment. This could be useful for differential therapy.