In order to find the reasons for the conflicting results depicted during the estradiol stimulations of cultured MCF7, breast cancer cells we investigated, besides cell counts, the cell surface transferrin receptor as an additional means of assessing the effect of estradiol. In this study we report results obtained using different culture conditions, i.e. short-term or long-term phenol-red withdrawn cells grown either in calf-serum supplemented media or defined media. Our results point out concurrent variations of cell counts and transferrin receptors when short-term phenol-red withdrawn cells were grown in defined media. Discrepancies were, however, observed when short-term phenol-red withdrawn cells were grown in serum-supplemented media or when long-term phenol-red withdrawn cells were grown in defined media. In both cases, only transferrin receptors account for estradiol stimulation. These results highlight the importance of transferrin receptor measurement in cultured breast cancer cell experiments and suggest cell kinetic perturbations due, in all likelihood, to serum factors or factors secreted by long-term phenol-red withdrawn cells.