The growth rate of stromal cells derived from 23 specimens of human endometrial tissue taken at various phases of the cycle was measured during the first seven days of primary culture. The stromal cells were found to differ radically in their ability to grow in culture according to the phase of the cycle at which the tissue was obtained. Early and mid proliferative stromal cells were capable of proliferation but that capacity was lost in late proliferative tissue. Growth at significant rates reappeared at days 21--22 and persisted in cells from late secretory tissue. Following plating, a 2--4 day lag period preceded proliferation of all but the most rapidly growing cultures. Subcultures showed a shorter lag period and faster maximal growth rate than the primary cultures from which they were derived. Estradiol, 10(-9) M. inhibition stromal cell growth in primary culture and this effect was prevented by 10(-8) M progesterone. These observations indicate that proliferative characteristics of stromal cells in primary culture are strongly influenced by the hormonal environment of the endometrium at the time of biopsy.