The cell cycle includes a series of metabolic events which progress through four distinct phases, operationally defined as G1, S, G2 and M. Since the mitotic phase is the only one visually distinguishable in the cycle, until recently, most of our knowledge about the cell cycle was obtained by indirect means such as autoradiography. The recent introduction of laser microfluorometry and microphotometry allows a faster and more quantitative analysis of cell kinetics. However, the various methods reported to date do not go beyond the acquisition of patterns of DNA distribution. Detection of growth fraction, the all or none phenomenon of DNA synthesis (G1-S boundary), the significance of GO cells, and phase segment specificity to a given antimetabolite (with consequent effects on progression through the cycle) are a few points which are extremely important in planning chemotherapy. They are still subject to debate. The objective analysis of images by the simultaneous measurement of up to 54 geometric and densitometric parameters per cell image makes a unique quantitative description of the morphology of nuclear chromatin possible. This technique, when combined with a new method developed in our laboratory for characterizing living cells by laser flow microfluorometry, can overcome the inherent limitations of both autoradiography and common pulse cytophotometry. We, therefore, propose to conduct a rigorous correlated study of the characterization of cell cycle phases (including GO) and kinetics (to include the effects of antimetabolites) by means of pattern analysis and densitometry, combined with laser microfluorometry and autoradiography using in vitro and in vivo systems.