We studied the isoenzymes of creatine kinase in the skeletal muscles of 63 patients with various neuromuscular diseases, of 30 human fetuses and of normal subjects (controls). In the muscle tissue of the control group the MM isoenzyme alone could be found; only 2 normals had traces of the MB isoenzyme as well (MB/ MM ratio = 0.09). During embryogenesis the distribution of isoenzymes in skeletal muscles gradually changes from the initial prevalence of the BB form, to the MB and before all, the MM forms. This shift is related to the development of contractile activity. The appearance of the MB isoenzyme, its relative augmentation and the increase of the MB/MM ratio are characteristic of the Duchenne and the other forms of progressive muscular dystrophy, as well as of acute polymyositis, the Kugelberg- Welander’s disease, dystrophic myotonia and lateral amyotrophic sclerosis. The observed changes are not specific for progressive muscular dystrophy. The appearance of the MB form and the increase of the MB/MM ratio is reminiscent of the CK isoenzymogram of 5- to 6-month human embryos. The observed phenomena may be correlated with a disturbance of the regulatory mechanisms.