Changes in the permeability of cell membranes and in the enzymatic activity of intact cells and their homogenates after irradiation were studied. CHO-cells, erythrocytes and their homogenates were irradiated with cobalt-gamma radiation and UV-light. It was found that the esterases are more radio-sensitive in intact cells than in the corresponding homogenates. The decrease of the enzymatic activity after irradiation can be described by a one-hit curve, whereas the loss of transport activity can be resolved into two one-hit curves. The slope associated with the more sensitive target mainly determines the course of the composed transport curve. Mechanisms for the decreases in the rate constants are discussed.