Tamoxifen is extensively used for the treatment of human breast cancer. However, the mechanisms by which antiestrogens regulate the growth of estrogen receptor positive tumors have not been totally defined. A new methodology, using automated image analysis BIOCOM 500, was developed for determining potential doubling time (Tpot) of tumors. This new method was checked on three different human breast cancer cell lines (MCF-7, CAL 85-1, CAL 148) in comparison with flow cytometry and then applied to determine the effects of short-term tamoxifen treatment on Tpot of MCF-7 cells. Using the resulting bivariate contour plot of blue fluorescence (DNA content) versus green fluorescence (Bromodeoxyuridine content), a labeling index (LI) value of 0.39 +/- 0.05 and a Tpot value of 21 +/- 2.09 hours were determined for MCF-7 cells. As expected, data demonstrated that 72 hours of 1 microM tamoxifen treatment decreased the LI to 35% by increasing the proportion of G0/G1 cells. It increased the Tpot to 35% compared to untreated cells (Tpot = 31.8 + 4 hours) by a lengthening of G0/G1 phase without changing the length of S phase (Ts = 10.2 +/- 1 hours). At suprapharmacological concentrations (5, 10 microM), an approximately 50% increase in Tpot was observed without modification in Ts. These data suggested a specific cell cycle action of tamoxifen which was probably mediated by mechanisms other than estrogen inhibition, since these experiments were performed in estrogen-deprived medium. In addition, the automated imaging procedure appears to provide a rapid and quantitative approach to determine Tpot in fine needle biopsies which is useful for investigating alterations in cell growth after endocrine treatment or chemotherapy.