Plasma MB creatine kinase (CK) activity was quantitatively evaluated by several independent methods in 9 young professional athletes after vigorous exercise in competitive football games. Seven of the 9 had elevated total plasma CK activity. None had clinical findings of myocardial injury. However, 4 of the 7 with total CK elevations also had elevated MB-CK. MB-CK elevations occurred in players with the highest elevations of total CK activity. It is known that well-trained athletes liberate less conventionally measured muscle enzyme into the blood after exertion than do untrained individuals. These results indicate that a substantial fraction of professional athletes who engage in unusually vigorous exercise have elevations of plasma MB-CK that should not be interpreted as necessarily indicative of myocardial ischemic injury.