Pyruvate kinase activity was examined in the sera of a group of patients with neuromuscular disease and in carriers, and compared with that of creatine kinase. The following observations were made: 1. Pyruvate kinase activity was elevated in all 14 patients with Duchenne muscular dystrophy, with very high values generally correlating inversely with age or disease duration. Elevated values of pyruvate kinase were usually, but not invariably, associated with elevated values of creatine kinase. 2. Almost all patients with other muscle diseases and those with neural atrophy had modest elevations in pyruvate kinase activity. 3. ten of 17 individuals were identified as carriers of muscle disease by using both pyruvate kinase and creatine kinase while eight and nine, respectively, were detected using either assay alone. 4. When frozen stored EDTA-plasma was used for pyruvate kinase estimation, higher levels, as compared with the corresponding sera or fresh plasma, were found in controls and carriers but not in Duchenne muscular dystrophy patients. Frozen stored EDTA-plasma should, therefore, not be used for diagnostic purposes.