The effects of various cytotoxic chemicals, as measured by viable cell counts, colony-forming ability and proliferative capacity, have been studied using Chinese hamster cells in exponential and plateau (stationary) phases of growth. The proliferating cells were altogether more sensitive to the action of the drugs than nonproliferating cells. However, imuran (azathioprine) a purine antimetabolite, was more effective against the plateau-phase cells. The observed response of cells to imuran could be detected at a wide range of concentrations (1–100 μg/ml). These findings are discussed in view of the possible ability of imuran to interfere with active metabolic processes in non-proliferating cells.