Germ cells harvested from mouse embryonic genital ridges were mixed with disaggregated embryonic lung cells, and the reaggregates were cultured for 4-7 days. Germ cells derived from female embryos 10.5-13.5 days postcoitum (dpc) entered and progressed through meiotic prophase in vitro as in vivo, although with a 12- to 24-hr delay. If the cultures were maintained for 2-3 weeks, the germ cells developed into growing oocytes. When germ cells were taken from male embryos 10.5 and 11.5 dpc, they too entered and progressed through meiotic prophase, but germ cells from later embryos (12.5 and 13.5 dpc) developed as prospermatogonia, as in male genital ridges in vivo. When 11.5 dpc male genital ridges were disaggregated, reaggregated, and cultured in the same way as the lung reaggregates, the germ cells again entered meiotic prophase. We conclude that the male genital ridge at about 12 dpc produces a factor that inhibits entry of germ cells into meiosis, and that production of this factor is disrupted by prior disaggregation of the genital ridge. If a meiotic inducing substance is required for entry of germ cells into meiosis, it must be present in the male genital ridge as well as in the female genital ridge, and probably also in the lung.