Cytokines such as interleukin (IL)-6 are proteins, which were originally discovered within the immune system. Recent studies, however, demonstrate that IL-6 is produced by, and released from contracting skeletal muscles during exercise. This release occurs in the absence of muscle damage and is related to both contraction per se and low muscle glycogen. IL-6 seems to work as an energy sensor within the muscle cells. Other organs also release IL-6 during exercise; however, muscle-derived IL-6 seems to play an important role in signalling between the muscles and other organs in order to maintain energy supply. IL-6 can enhance lipolysis in humans and might play a role in glucose metabolism. In addition, muscle-derived IL-6 is likely to initiate many of the exercise associated immune changes, as IL-6 can increase plasma levels of the cytokines IL-1 ra and IL-10, together with cortisol and blood neurtrophils. Also, the observed shift towards Th2 lymphocyte dominance during exercise may be mediated by IL-6. Because carbohydrate ingestion during exercise has been demonstrated to blunt the IL-6 and hormonal response, it might also blunt other beneficial adaptations. This review discusses the possible beneficial biological role of high plasma levels of IL-6 during exercise.