Serum creatine kinase isoenzymes were studied in 41 patients suffering from Duchenne type muscular dystrophy and 20 mothers of patients (carriers) by cellulose acetate electrophoresis. Both the MM and MB types were found in all cases of Duchenne type dystrophy patients, and in carriers with highly elevated total creatine kinase activity BB was not observed above the detection limits of the methods used. However, a so-called atypical CK--BB band has been demonstrated.