Interleukin-6 bioactivity (IL-6) has been shown to be present in Sertoli cells. To further characterize the IL-6 in the seminiferous epithelium, the IL-6 like-antigen was detected, stage-specific basal distribution of IL-6-like bioactivity and its regulation by FSH, cAMP and TPA was characterized in isolated, rat seminiferous tubule segments. In addition, the effects of human recombinant IL-6 on stage-specific DNA synthesis was investigated. Both monoclonal and polyclonal antibodies recognized M(r) 22 and 23 kDa of IL-6 like immunoreactivity in the seminiferous epithelium. The basal IL-6 production showed high levels during stages XIII-XIV-I-V, low during VII and VIII. FSH stimulated IL-6 production at nearly all stages and most significantly at stage VII of the cycle. Human recombinant IL-6 dose-dependently inhibited the onset of meiotic DNA synthesis of preleptotene spermatocytes, and a minor inhibition was found on advanced (A3-type B) spermatogonia. These results support the hypothesis that IL-6 is a stage-specific paracrine regulator of the seminiferous epithelium exerting a specific inhibitory action on meiotic DNA synthesis.