Recently described techniques for separating myosin isoenzymes have been adapted for analysis of myosins from diseased and developing human skeletal muscle. The method is highly suitable for analysis of human myosins because only 2 - 3 mg of muscle are required for routine analyses. Human embryonic/foetal myosins are electrophoretically distinct from mature skeletal myosins, and are not normally detected beyond the first month of post-natal life, except in premature infants. They have a high alkaline calcium-activated ATPase activity. This would account for the histochemical classification of foetal fibres as "Type II", although physiological differences between adult fast-twitch muscle and foetal muscle are well recognized. Foetal myosins are also synthesized in human skeletal muscle under certain pathological circumstances. Their presence in Duchenne dystrophy probably reflects the associated marked muscle regeneration, with immaturity of some muscle cells. The large amounts of foetal myosin present in many cases of infantile spinal muscular atrophy is evidence that innervation is necessary for the normal cessation of foetal myosin synthesis.