GONADOTROPHINS and various growth factors, but not sex steroids, can induce resumption of meiosis in vitro, but only in oocytes enclosed by cumulus-granulosa cells¹. Follicular purines prevent resumption of meiosis^2,3. This process can be overcome, in vitro, by a transient elevation of cyclic AMP resulting in the production of a diffusible meiosis-inducing substance secreted by the cumulus cells⁴. A meiosis-inducing activity has been detected in gonads of different species, for example, in preovulatory follicular fluid of women⁵ and in mouse testes⁶. We report here the isolation and characterization of meiosis-activating sterols from human follicular fluid and bull testes and the synthesis of two closely related C²⁹ sterols. AH these sterols induce a resumption of meiosis in cultured cumulus-enclosed and naked mouse oocytes indicating their nonspecificity across species and sex. This family of sterols is for the first time considered crucial to meiosis.