Variations in the number of peripheral burst-forming unit--erythroid (BFU-E) of healthy women were observed during a prolonged period of observation. These differences were related to different phases of the menstrual cycle. A peak in the number of BFU-E occurred on day 14 of the cycle corresponding to the serum 17-beta-estradiol peak. The effect of estrogens and progesterone on the in vitro growth of peripheral BFU-E of healthy women was assayed. Estrogens demonstrated a stimulatory and progesterone an inhibitory effect in total lymphomonocyte cultures, whereas neither hormone had an effect in monocyte-depleted cultures. Prostaglandin E1 (PGE1) which is known to be secreted by monocytes, stimulated the in vitro growth of peripheral BFU-E. These data suggest that estrogens and progesterone could have a role in the in vitro growth of peripheral BFU-E, probably mediated by monocytes.