Quantification of the proliferative characteristics of normal and malignant cells has been of interest to oncolo- gists and cancer biologists for almost three decades. This interest stems from (a) the fact that cancer is a disease of uncontrolled proliferation, (b) the finding that many of the commonly used anticancer agents are preferentially toxic to cells that are actively proliferating, and (c) the observa- tion that significant differences in...