The contractile proteins present in muscle biopsies taken from infants suffering either from congenital myotonic dystrophy or X-linked myotubular myopathy were compared using biochemical and immunocytochemical techniques. Two-dimensional gel analysis has revealed that in all cases of X-linked myotubular myopathy the pattern of expression of myosin light chains, tropomyosin and troponin was roughly similar to that of normal age matched control muscle. However, biopsies from infants affected by congenital myotonic dystrophy demonstrated a predominance of most fast contractile protein isoforms. Non-denaturing gel electrophoresis confirmed the presence of both fast and slow myosin isoforms in X-linked myotubular myopathy. Fetal myosin was also present but in amounts higher than that found in normal muscles of the same age. In congenital myotonic dystrophy fetal and fast myosin were the predominant isoforms detected by native gel electrophoresis. These results were confirmed by immunocytochemistry and Western blot analysis using antibodies specific for the different myosin isoforms.