Two pregnancies at risk for X-linked recessive Duchenne's muscular dystrophy were studied at 18 and 20 weeks. Fetal blood was obtained by placental aspiration for measurement of plasma creatine phosphokinase activity. Activity in the first fetus was 96 IU per liter, as compared to a control range of 0 to 150 IU per liter in 16 pregnancies not at risk for the disorder. The pregnancy continued, and the infant was normal after birth. In the second fetus creatine phosphokinase activity was significantly elevated to 540 IU per liter (P less than 0.001). Fetal blood also showed considerable hemolysis, an unusual observation in placental blood sampling. After abortion, examination of fetal muscle by light, phase and electron microscopy showed characteristic features of Duchenne's muscular dystrophy, including wide variation in muscle-fiber diameter and reduction in the number of fibers per fasciculus. These cases illustrate the potential usefulness of fetal plasma for prenatal diagnosis and, specifically, of creatine phosphokinase activity for diagnosis of muscular dystrophy.