B lymphopoiesis is selectively suppressed in normal pregnant mice, suggesting that fluctuations in systemic hormone levels might influence local events within bone marrow. This has now been tested by sustained experimental elevation of sex steroids by hormone-containing pellet implants. We found that while numbers of total nucleated cells declined after treatment with estrone, beta-estradiol, or estriol, there was preferential suppression of B-lymphocyte lineage precursors. Progesterone pellets had no effect when used alone, but mice exposed to progesterone were sensitive to several-logarithm lower concentrations of estrogen. Changes in subpopulations of B-lymphocyte lineage cells with hormone pellets were similar to those previously recorded in pregnancy. B-lymphocyte lineage precursors in male and female mice were sensitive to these sex hormones. Acute treatment with single injections of water-soluble beta-estradiol allowed temporal effects on B-lineage cells to be documented. With this protocol, total numbers of nucleated cells and myeloid progenitor cells remained unchanged. Interleukin 7-responsive precursors dramatically declined within 1 day of injection, suggesting that estrogen influences that stage in the B-lymphocyte lineage. There was a subsequent sharp drop in small pre-B cells 4 days after this transient elevation in estrogen. These experiments demonstrate that B lymphopoiesis is sensitive to negative regulation by sex steroids. They extend findings made with pregnant animals and parallel previous studies of the thymus. Sex steroids might contribute to control of steady-state lymphopoiesis, and fluctuations in their levels could have implications for human disease.