The transformation of UV-C irradiated leucocytes and lymphocytes by the mitogens Con A, PHA and PWM is measured by the 3H-Tdr. incorporation after 72 h incubation. Furthermore T- and B-cells are determined by the method of rosette formation. A clear inhibition of the cell activity is seen after irradiation of leucocytes with 450 mJ/cm(2) and of a lymphocyte suspension with 5-10 mJ/cm(2). There are no significant differences between the effects of the various mitogens. The number of T-cells decreases proportionality to the various intensity, the number of B-cells remain constant. Irradiation with UV-A + 8-MOP cause, equally for all mitogens, a dosis dependent inhibition of thymidine incorporation.