Abstract: Based on the discovery that normal pregnant mice have a striking reduction in committed precursors of B lymphocytes, which could be documented in mice as early as day 6 of gestation, when IL-7 responding colony forming cells were reduced as much as two-thirds of normal levels, it has been determined that estrogen and other hormones elevated in pregnancy induce a specific modulation of lymphocyte formation during pregnancy and lactation. It is therefore possible to immunomodulate in a specific manner an animal by administration of hormones elevated during pregnancy, such as estrogen and estrogen-like compounds, or antagonists of estrogen. This has potential in the treatment of a number of disorders, especially those found in very high percentages of women as compared with men, such as many of the autoimmune disorders, as well as in immune tolerance during pregnancy, cyclic neutropenia, and osteoporosis.