gamma-Glutamyl transpeptidase, a membrane-bound enzyme playing an important role in the active amino acid transport across cellular membranes, is shown to be elevated in the serum of patients with myotonic muscular dystrophy. No increase of AP, LAP, GOT and GPT activities in the sera of some of the patients studied is observed. Possible interpretations in relation to the pathogenesis of myotonic dystrophy are discussed.