Leukemia inhibitory factor (LIF) stimulates the growth of primordial germ cells (PGCs) in mouse embryo. However, as neither mice lacking LIF nor mice lacking the LIF receptor show defects in PGC growth, an alternate cytokine for PGC growth has been postulated. We investigated the role of mouse oncostatin M (mOSM), which is structurally and functionally related to LIF, in germ cell development. While LIF enhanced the survival of migratory as well as postmigratory PGCs, mOSM acted only on the postmigratory PGCs. Consistent with its biological activity, mOSM was found to be expressed in developing gonads. In the male, Sertoli cells in neonatal testis express mOSM; however, its expression is downregulated in adult testes. Moreover, mOSM enhanced the proliferation of Sertoli cells derived from neonatal testesin vitromore than human OSM or LIF. In contrast, postnatal ovaries do not express mOSM. These results indicate that mOSM is a stage- and sex-specific autocrine growth factor for Sertoli cells.