Serum creatine kinase (SCK) was measured in ten subjects in the laboratory before and after the performance of bicycle ergometry and a lifting task. SCK was significantly increased 24 h and 48 h after the lifting work but not after the bicycle ergometry, although the work performed on the latter was four-times as great as on the former. The lifting work resulted in muscle pain and tenderness and, for six subjects, in clinical signs of shoulder tendinitis. In a field study, an increase in SCK was noted among assemblers/welders and cash-register operators, but not among controllers and forklift-truck drivers. A health interview revealed that musculo-skeletal complaints were most often located in the upper extremity in the cases of the assemblers/welders and the cash-register operators. It is proposed that the SCK increase during work is a result of a high local muscular load due to fatigue and energy depletion of muscle cells producing a greater efflux of muscle enzymes. The evaluation of SCK changes during work may be an important tool in occupational health for early detection of work tasks producing local muscular strain.