Two enriched populations of Leydig cells (hLCI and hLCII) have been characterized in human testes; it is noteworthy, that in the presence of hCG the steroid ouput is higher in hLCII when compared to hLCI; conversely, the basal production of steroids is greater in hLCI than in hLCII. The addition of increasing amounts of seminiferous tubule-conditioned medium to the purified Leydig cells leads to a dose-related enhancement of the steroid production in both Leydig cell fractions under basal and hCG-stimulated conditions. It is therefore obvious that a paracrine factor (or factors) from seminiferous tubular origin influences positively and with a high efficiency the human Leydig cell function. The human Sertoli cell synthesizes lactate, estradiol-17beta and several proteins, namely transferrin, ferritin and inhibin. In the presence of germ cells the Sertoli cell production of estradiol-17beta is decreased whereas the transferrin and inhibin outputs are enhanced. In addition the lactate dehydrogenase, gamma-glutamyl transpepetidase, alkaline phosphatase and creatine phosphokinase activities have been quantitated in various human Sertoli cell preparations. It should be kept in mind that germ cells exert an important influence on the adult Sertoli cell secretory activity through either direct contact and/or via secreted factors; moreover germ cells potentialize the stimulating effect of FSH on the Sertoli cell function and indirectly the Leydig cell output of testosterone via the Sertoli cell secretion of paracrine factor(s).