The antiproliferative activity of the antiestrogen, tamoxifen, on the growth of MCF-7 human breast cancer cells was evaluated using the hemocytometric trypan blue exclusion method, [3H]-thymidine incorporation, and a total protein determination. Tamoxifen was evaluated over a concentration range from 10(-9) to 10(-6) M. The hemocytometric trypan blue exclusion method and [3H]-thymidine incorporation were sensitive enough to demonstrate the inhibitory influence of tamoxifen on the proliferation of MCF-7 cells at a concentration as low as 10(-9) M. A very good correlation of these two methods was observed in the submicromolar concentration range of tamoxifen. The total protein determination method was only sensitive enough to detect the antiproliferative influence of tamoxifen at concentrations above 10(-6) M. In conclusion, the [3H]-thymidine incorporation method was found to be effective and much less time consuming than the hemocytometric trypan blue exclusion method for evaluating the antiproliferative effects of antiestrogens in cultured MCF-7 cells. However, when evaluating antiestrogens, which are cell-cycle specific, the results of the [3H]-thymidine incorporation method should be interpreted with caution.