Muscular contraction, especially if it is protracted or particularly intense, causes notable changes in the substrate and enzymes involved. Marked changes have been found in the serum levels of myoglobin and CK in individuals subjected to very stressful physical work. This phenomenon has usually been attributed to the repeated and intense muscular traumas inherent in certain sporting activities. In the light of these facts, the authors decided to perform a study on amateur boxers aimed at measuring the difference between simple muscular work (shadow boxing) and traumatic activity (real boxing) in terms of their effect on a number of humoral parameters, especially myoglobin and CK. The serum levels of these two latter substances were found to increase more after real boxing than after shadow boxing.