The appearance of β-enolase, a glycolytic enzyme, was studied immunohistochemically using the upper limb bud of human embryos at Carnegie stages from 13 to 21. β-Enolase-immunoreactive cells first appeared at stage 15 in the proximal portion of the upper limb bud. It was evidenced that glycogen granules first appear at the same stage. These results may suggest that changes in energy metabolism might be one of the earliest events in the differentiating steps of the skeletal muscles because this stage is earlier than the stages of cell fusion myofilament formation and innervation of the muscle cell