Quadriceps muscle samples from human fetuses (10 weeks of gestation to term) were studied using immunocytochemical methods with monoclonal antibodies against fetal, adult-slow and adult-fast B myosin heavy chains. The monoclonal antibodies were selected for their virtually exclusive specificity for a particular isomyosin and used to investigate the expression of different myosin heavy chains during fetal development of muscle fibres. Concomitant studies of the myofibrillary ATPase pattern of muscle fibres were carried out. A fetal-specific myosin was persistently expressed during fetal life but at a continuously decreasing rate. Adult-slow myosin was observed in a small pool of muscle fibres, histochemically undifferentiated, in fetuses of 14-16 weeks of gestation. However, adult isomyosins appeared intensively only in the late fetal period, progressively replacing fetal myosin. The genes coding for adult-slow myosin are expressed earlier that those coding for adult-fast myosin. Myosin heavy chains specific for the neonatal period were not demonstrated with the antibodies used in this study. The contribution, provided by the present study, to the knowledge of the sequence of events in the expression of myosin heavy chains during normal muscle development, may allow a better understanding of eventual myosin changes which may occur in genetic muscle disorders.