The dependence of the surviving fraction of Chinese hamster cells in vitro upon the concentration of Thio-TEPA, upon the duration of drug exposure and upon fractionation of drug exposure has been studied. Given a constant exposure time, the surviving fraction decreased according to a "shoulder" curve with increasing concentration. Given a constant concentration, the surviving fraction decreased exponentially with increasing exposure time. Splitting exposure of 60 min into two 30 minutes' exposures with drug-free intervals of several hours increased the cytostatic effect of Thio-TEPA.