By using immature porcine Sertoli cells cultured in serum-free defined medium, we report that medium conditioned by Sertoli cells contained immunoreactive somatomedin C/insulin-like growth factor 1 (SmC/IGF1) measured following acidic gel filtration. The release of this immunoreactive SmC/IGF1 was slightly increased following Sertoli cell treatment with fibroblast growth factor but not with follicle-stimulating hormone or growth hormone. On the other hand, human biosynthetic SmC/IGF1 exerts a potent stimulatory effect on Leydig cell differentiated functions such as LH/hCG-binding (greater than 4-fold) and hCG-stimulated testosterone secretion (greater than 15-fold). This effect was dose and time dependent and the maximal increase of Leydig cell function was observed following 48 h treatment with 50 ng/ml SmC/IGF1. The steroidogenic action of the peptide was not related to Leydig cell growth since both cell number and 3H-thymidine incorporation into DNA were not or slightly (approximately equal to 1.5-fold) increased in the optimal conditions with SmC/IGF1 treatment (100 ng/ml for 48 h). Moreover, the concomitant treatment of Leydig cells by both arabinoside C (10(-5) M), a DNA synthesis inhibitor, and SmC/IGF1 did not modify the stimulating effect of the peptide on LH/hCG-binding and hCG-stimulated testosterone production. Taken together, the present findings support the concept that Sertoli cell derived SmC/IGF1 could be a potent regulator of Leydig cell differentiated functions.