One of the factors of importance in determining the killing of mammalian cells following exposure to a variety of anticancer agents is the proliferative state of the cell population. Generally, proliferating cells are much more sensitive to anticancer agents than are nonproliferating cells. In this review, the cellular aspects of this differential sensitivity are discussed with the hemopoietic stem cell population and tissue culture cells as the focus for the analysis. This phenomenon is not only of concern to the cell biologist but also has implications with regard to scheduling of anticancer agents against human tumors.