We have studied the effects of human interleukin-1 beta on steroidogenesis in cultured immature rat Leydig cells. In the presence of low concentrations of LH or in its absence interleukin-1 beta markedly stimulates the production of C19-steroids (testosterone and androstenedione) and C21-steroids (progesterone, 17 alpha-hydroxyprogesterone, 20 alpha-hydroxypregn-4-en-3-one). In the presence of maximally effective concentrations of LH, on the contrary, interleukin-1 beta inhibits C19-steroid production by provoking a block at the level of the 17,20-desmolase. These actions were observed with similar low doses of interleukin-1 beta (ED50 = 1 U/ml), but the stimulatory effects are evident within the first 2 h of incubation whereas the inhibitory actions appeared after a latent period of 6 h. None of the effects of interleukin-1 beta is accompanied by measurable changes in cAMP output, and the effects are much less pronounced in freshly isolated Leydig cells than in cultured cells. At maximally effective doses the effects of interleukin-1 beta are additive with those of a number of other Leydig cell agonists: LHRH, epidermal growth factor, arginine vasopressin and Sertoli cell-derived factor(s), suggesting that these agonists act by mechanisms different from that of interleukin-1 beta. The possibility is considered that Leydig cells may act as target cells for interleukin-1 beta derived from testicular macrophages or for interleukin-1-like factors derived from testicular tubules.