In this paper we describe the microcomputer-aided determination of cell proliferation kinetics and doubling times utilizing a crystal violet assay and a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide assay in microtitration plates. The analysis of spectrophotometric data provides the doubling times at any time of incubation. Plots of doubling time versus time of incubation give reproducible information on the exact duration of the logarithmic growth phase. This method is applicable to anchorage-dependent as well as anchorage-independent cells when colorimetric or fluorometric data are accessible.