Creatine kinase isoenzyme BB (CK-BB) is found in high concentrations in the brain. Normally concentrations in blood are undetectable or low. Blood concentrations of CK-BB were measured in 16 boxers before and after a fight and in 16 track cyclists before and after a race. Blood CK-BB rose to significantly higher concentrations in the boxers than in the cyclists. The number of blows received to the head was estimated in half the boxers and correlated significantly with the rise in CK-BB. This increase in blood CK-BB may indicate disruption of the blood-brain barrier. This may be one of the mechanisms accounting for brain damage in boxers.