For proliferating cell systems in which the usual "labeled mitoses" method cannot be used to estimate generation times, an alternative scheme is derived. The method presented here is based on observation (by autoradiography) of the median grain count of labeled interphase cells following a pulse of labeled DNA precursor. It is shown that the median generation time of the labeled cells will be equal to the time required for the median grain count to halve, starting from the time when half the labeled cells have completed their first division. This starting time is determined from observation of the first wave of labeled mitoses. The procedure was designed to minimize error resulting from such factors as radiation damage, label reutilization, and the use of a nonzero grain counting threshold. The method is applied to the analysis of two cases of acute leukemia in man.