The single intramuscular injection of 275 mg Rolitetracyclin (Reverin) led to a rise in serum creatine phosphokinase in 11 out of 20 heart healthy patients, 7 cases with values over 100 mU/ml. In sane cases the initial values had still not been reached 72 hours after injection. With Rolitetracyclin given intravenously the creatine phosphokinase values do not alter, as with an isotonic Na-Cl solution given intramuscularly. A rise in the serum creatine phosphokinase was seen in 2 out of 6 cases after an intramuscular injection of Oxytetracyclin (Terramycin -Depot). One is not dealing with a reaction which is typical only to Rolitetracyclin. The cause is thought to be the setting free of enzymes through the musclelesions. The results underline the sensitivity of and problems involved with creatine phosphokinase in the diagnosis of heart-infarction.