A simple analytical method is described for the evaluation of flow cytometric data of perturbed cell populations obtained by applying the BrdUrd-33258 Hoechst technique. This procedure allows for determining the growth curve, the efflux from and the influx into G2 + M as well as the transit characteristics of cell cohorts through the G2 + M phase. As an example of the application of the method, the radiotoxic effects of incorporated 3H-thymidine on the proliferation of L-929 cells in vitro are described.