Concanavalin A-stimulated splenocytes secrete a factor that stimulates progesterone production in cultured rat granulosa cells. The actions of this progesterone-stimulating factor (PSF) were characterized further by purifying it by sequential chromatographies on heparin-agarose, copper chelating-Sepharose and Mono S columns. Several of its effects on granulosa cells were then compared with those of FSH. Like FSH, PSF induced accumulation of progesterone and 20α-dihydroprogesterone in granulosa cell culture media; however, the time course of progesterone accumulation in response to PSF was much slower than that in response to FSH. In contrast to FSH, PSF induced little accumulation of oestrogen. Induction of other differentiated responses was compared by pretreating cultured granulosa cells with either FSH or PSF, and then challenging with LH, prolactin, epidermal growth factor or the β-adrenergic agonist, isoproterenol. Pretreatment with FSH induced subsequent responsiveness to all four agents, whereas pretreatment with PSF induced subsequent responsiveness only to isoproterenol. These results indicate that granulosa cell responses to PSF are much more limited than are those to FSH and that these responses are characterized by an increase in progestin production.