The beneficial health effects of exercise might be related to keeping the body s energy balance in place, or they could be explained by peptide signals released in response of contractional activity in the skeletal muscles. Secretory muscle-derived peptides are recently named myokines and might exert beneficial paracrine or endocrine effects. Today the list includes interleukin (IL)-6, 8, and 15, but the list is increasing rapidly as shown in the present thesis The aim of this study was to investigate the secretion and regulation of known and novel myokines in primary cultures of human skeletal myotubes. By the use of spin-columns, 1-D gel and MALDI-TOF, we used a method sensitive enough to identify novel proteins secreted from myotubes. A High Sensitivity Human Cytokine Lincoplex kit was also used to identify novel myokines. Lastly, different experiments were performed to examine to role of glucose, insulin, foetal calf serum, fatty acids, and ligands for nuclear receptor as well as toll like receptor (TLR) in regulating myokine secretion. We show that cultured myotubes have the potential to secret albumin, haptoglobin, immunoglobulin (Ig) alpha-1 chain C, Ig gamma-1 chain C, Ig kappa chain C, Ig kappa chain V-3, retinol-binding protein, transferrin, interleukin (IL)-6, IL-7, IL-8, granulocyte macrophage colony-stimulating factor (GM-CSF) and tumour necrosis factor alpha. Furthermore, our findings show that the secretion of IL-6, IL-8, GM-CSF and Tumour necrosis factor alpha is regulated by TLR ligands. In conclusion, cultured human skeletal muscle cells produce and secrete a large variety of peptides and might be an important part of the innate immune system.