Determination of the creatine kinase isoenzyme pattern in 62 biopsy samples obtained from patients with neuromuscular disease revealed changes mainly in Duchenne muscular dystrophy. The BB isoenzyme was detected in 10 out of 17 cases with Duchenne muscular dystrophy and the relative amount of MB+BB isoenzyme was significantly increased in this group (P less than 0.005). In serum the MB isoenzyme was detected in all 28 cases with progressive muscular dystrophy and frequently also in other neuromuscular diseases. Among 152 control samples the MB isoenzyme was detected only in 2 cases. It is suggested that the finding of MB isoenzyme in the serum with normal or only slightly elevated total CK activity may be a further proof of neuromuscular disorder, but the finding is not specific for any particular disease.