There is considerable evidence that Sertoli cell function is controlled not only by hormones, but also by locally produced growth factors and cytokines. To gain more insight into the nature and effects of cytokines potentially involved in the control of Sertoli cell function, we incubated rat Sertoli cells with media conditioned by activated human peripheral blood mononuclear cells. Such media (PBMC-CM) are known to be an extremely rich source of a variety of cytokines. It was demonstrated that PMBC-CM and protein fractions derived from them stimulate Sertoli cell transferrin secretion and messenger RNA production more potently then peritubular cell-conditioned medium or FIRT (a combination of FSH, insulin, retinol, and testosterone). Transferrin secretion expressed per mg cell DNA was stimulated approximately 5-fold by peritubular cell-conditioned medium or FIRT and nearly 20-fold by PBMC-CM. The effects of PBMC-CM were accompanied by a limited increase in cAMP and a noticeable rise in cGMP. Affinity chromatography on a column coated with an antiserum directed against interleukin-1 beta (IL-1 beta) showed that part of the activity in the PBMC-CM was related to IL-1 beta. The remainder of the activity was largely retained by an affinity column coated with an antiserum that recognizes IL-6 and a number of other known and unknown cytokines. Purified IL-1 beta provoked a 2- to 3-fold stimulation of Sertoli cell transferrin secretion. More limited stimulatory effects were observed with IL-6. Neither of these cytokines or their combination approached the degree of stimulation observed with crude PBMC-CM, suggesting that other cytokines are involved. It is concluded that the mixture of cytokines present in PBMC-CM is a more powerful stimulator of Sertoli cell transferrin secretion than any other agonist known at the present time. IL-1 and IL-6 may be responsible for part of the observed effects, but one or more other cytokines are probably involved.