Duchenne muscular dystrophy is a severe inherited disease. The pathogenesis is unknown. Duchenne dystrophy is characterized by a large number of membrane abnormalities, which are manifested by a leakage of muscle enzymes, such as creatine kinase (CK), and a reduction in cap formation in lymphocytes. In the present study serum CK and percentage lymphocyte capping in 8 patients with progressive muscular dystrophy, 6 with different myopathies and normal controls are investigated. Reduced antibody-induced redistribution of membrane antigens of B and T lymphocytes and high serum CK activity is found in boys with Duchenne dystrophy if compared with normal subjects and no correlation exists between the two parameters. Our data indicate that the determination of fetal serum CK activity associated with fetal lymphocyte capping may have diagnostic value in antenatal detection of Duchenne dystrophy.