Natural sulfated polysaccharides (SPs) derived from brown seaweed comprise a complex group of macromolecules with a wide range of important physiological properties. SPs have been shown to bind and directly regulate the bioactivity of growth factors and cytokines such as basic fibroblast growth factor, interferon, various enzymes and transforming growth factor. Myostatin is a member of the transforming growth factor-beta (TGF-beta) family that acts as a negative regulator of skeletal muscle mass. In this work we demonstrated that SPs isolated from the brown seaweed Cystoseira canariensis bind to the myostatin protein in serum.