With monoclonal antibodies (Mab) specific for myosin heavy chain (MHC) isozymes, we have investigated the isomyosin content of atrial, ventricular and conductive fibers of 19 human fetuses (ranging from 14-36 weeks of gestation) and 3 newborns (2 days-2 weeks). In addition, the conduction system of 2 human adult hearts was studied. The fetal atrium is composed mostly of α-MHC during the first 23 weeks of gestation. β-MHC is already expressed as traces at 14 weeks of gestation, and its expression increases progressively until birth, resulting in a great augmentation in β-MHC. During this course, β-MHC always predominates in certain areas (the crista terminalis and the interatrial septum) but not in other areas (the auricles). Preceding birth, the fetal ventricle is composed mostly of β-MHC. From 14 weeks of gestation to birth, α-MHC is expressed in very rare fibers. Then, after birth, a large number of fibers simultaneously synthesize α-MHC. The AV node and His bundle system were labelled with anti-α and anti-β Mab in fetal, newborn, and adult hearts with a double gradient of distribution: spatial (a higher proportion of α-containing fibers in the AV node than in the distal portion of the bundle of branches) and temporal (a higher proportion of α-containing fibers at a given point in fetal development than in the adult heart). One of the twenty-five hearts studied had an isomyosin distribution pattern not accorded to its age. Interestingly, it was clinically diagnosed as having idiopathic hypertrophic cardiomyopathy.