The enzymes ALD, CPK, GOT, CPT, LDH and MDH were studied in the serum of 221 patients suffering from progressive muscular dystrophy and in 43 cases of infantile and pseudomyopathic muscular dystrophy. Besides, the activity of 25 enzymes was measured in the muscular tissue. All findings were statistically revised. Serum enzymes are increased in all of the 3 types of muscular dystrophy; in most cases of the Duchenne type, however, many times over the other types. The difference is not specific, but can be explained by the considerably more marked progression of muscle degeneration in the Duchenne type. The enzyme increases are already existent at birth and reach a maximum before clinical manifestation of the disease. The activities of all serum enzymes decrease with longer duration of the disease. This phenomenon also occurs in the majority of all serum enzymes. Up to now, modifications indicating a specific abnormality of metabolism could not be stated. Likewise in spinal muscular atrophies of the Kugelberg-Welander type, there appear the same enzyme losses in the muscle and significant increases of the enzyme activities in serum can be demonstrated. This does not apply to the infantile forms of Werdnig-Hoffmann type and this fact might be explained by the almost complete motorial inactivation. The hypothesis that the efflux of enzymes from the muscular cell might point to a disordered membrane permeability as initial process of muscular dystrophies, is likewise refuted by finding necrotic muscular fibers, already in the earliest preclinical stage of the disease. The serum enzyme findings are of limited value for establishing a clinical differential diagnosis. Only extremely marked increases of activity do reliably point to the Duchenne type.