Interleukin-6 (IL-6) was produced by the spontaneously immortal Schwann cell clone, iSC, when cocultured with PC12 cells. The iSC cell-derived IL-6 in coculture conditioned media caused the neuronal differentiation of naive PC12 cells and this bioactivity was neutralized by preincubation of conditioned media with antisera to IL-6. Cocultured iSC transcribe IL-6 message as confirmed by northern analysis. Stimuli that induce IL-6 production in the hematopoietic lineage induced transcription and production of IL-6 by iSC cells. Lipopolysaccharide, tumor necrosis factor-alpha, IL-1 alpha, IL-6, and serum withdrawal induced iSC cell IL-6 mRNA. The kinetics of IL-6 production was confirmed in the mouse IL-6-dependent B9 bioassay and that activity could be neutralized with antisera to IL-6. Expression of both the IL-6 receptor and the gp130 signal transduction component by iSC as determined by northern analysis suggests an autocrine regulatory mechanism. The observed iSC production of IL-6 in vitro led to an investigation of the sciatic nerve crush model of Schwann cell activation in vivo. In the initial 12 h after crush injury, IL-6 message is induced. IL-6 mRNA expression was highest distal to the crush injury. Our in vitro data demonstrate that iSC cells produce IL-6 in response to PC12 cell coculture and to stimuli that induce IL-6 production in the hematopoietic lineage. The induction of IL-6 message distal to a crush injury suggests another mechanism by which Schwann cells facilitate peripheral nerve regeneration.