Carbonic anhydrase III and myoglobin concentrations were measured from vastus medialis muscle samples before and 6 weeks after postoperative leg immobilization following knee ligament reconstruction. Seven patients were immobilized with conventional cast so that the knee joint was fixed at 20° flexion. Seven other patients used braces allowing a motion of 30–70°. The atrophy of the medial part of musculus quadriceps was estimated by computer tomography. No difference in carbonic anhydrase III and myoglobin concentrations or muscle atrophy between the cast and brace groups was observed. The cross-sectional area of the medial part of musculus quadriceps decreased 38% during immobilization. The specific concentrations of carbonic anhydrase III and myoglobin remained unaltered. Due to the remarkable postoperative atrophy of the muscle the total carbonic anhydrase III and myoglobin contents, however, decreased significantly (37% and 31%, respectively). The results suggest that the net breakdown of carbonic anhydrase III and myoglobin during disuse atrophy occur at the same rate as the average net degradation of mixed muscle proteins and that the process is independent of the immobilization procedure.