The surviving fraction (SF) of Chinese hamster cells and HeLa cells after treatment with a range of Adriamycin concentrations and exposure times was determined. The cytostatic effect was proportional to the product of extracellular drug concentration (c) and exposure time (t) according to the equation: SF=e-ktc. By determining the intracellular drug concentration at various exposures, it could be shown that absorbed dose is not proportional to exposure dose.