High serum concentrations of estradiol (E2) equivalent to those observed in the luteal phase of the menstrual cycle stimulate both epithelial cell proliferation and progesterone receptor (PgR) expression in normal human breast tissue xenografted into athymic nude mice. We report here the results of further investigations designed to determine whether the induction of PgR expression and proliferation require different E2 concentrations and whether proliferating cells expressed the PgR. In untreated normal breast xenografts, the PgR was virtually undetectable and proliferation was at basal levels. Progesterone (Pg) treatment alone had no effect compared to no treatment. Treatment with E2 at follicular phase serum concentrations maximally increased PgR expression but was without effect on proliferation. However, treatment with E2 at luteal phase serum concentrations, alone or in combination with Pg, significantly increased both the PgR content and the proliferation of the breast epithelium. These experimentally derived data reflected the observations made on normal breast tissue at surgical biopsy where PgR content was similar in both halves of the menstrual cycle, whereas proliferation was significantly higher in the luteal phase. Finally, using double labelling techniques, it was demonstrated that proliferating epithelial cells rarely expressed PgR in normal breast tissue obtained at surgical biopsy. These results suggest that the threshold of E2 required to induce PgR expression in normal human breast epithelial cells is lower than that required to induce proliferation and that the majority of proliferating breast cells do not express the PgR.