1. Myosin was isolated from human right- and left-atrial and -ventricular myocardium, and examined both in adult subjects and at different stages during pre- and post-natal development. 2. The myosin light-chain subunits in the atria and ventricles were different when characterized by isoelectric focusing and subsequent two-dimensional poly-acrylamide-gel electrophoresis. 3. No differences were observed between the light-chain subunits in the right and left ventricle at any stage of development. 4. The foetal ventricle contained a characteristic light chain that was a major component throughout the latter half of gestation. This foetal light chain, which disappeared in the postnatal period, could not be distinguished from adult atrial light chain 1 on two-dimensional electrophoresis. 5. Myosin in the adult atria, particularly the left, contained components similar to ventricular light-chain components. 6. The possible stimuli for the observed changes in myosin light-chain expression are discussed in relation to the known physiological changes occurring during development.
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