Commercial control preparations were exposed to light at room temperature for as long as 24 h. Decreases in creatine kinase activity in seven of the nine controls ranged from 28.8 to 83.3%. When these controls were protected from light and stored at various temperatures, the decrease in activity was either eliminated or substantially reduced, indicating that the decreased activity in the presence of light was not due entirely to thermal inactivation. In the absence of oxygen, light had little effect on creatine kinase suggesting that light inactivation is a light-catalyzed oxidative process. The decreased activity observed when some of the controls were exposed to light was not completely restored by incubating with dithiothreitol before analysis. However, increases in creatine kinase activity ranging from about 37 to 176% were observed when freshly prepared controls were pre-incubated with dithiothreitol. Neither light nor dithiothreitol had any effect on the creatine kinase activity in two of the nine controls. When human sera were exposed to light for 24 h, the largest decrease in activity was about 15%. Incubation of fresh human sera with dithiothreitol before analysis caused an average increase in activity of approximately 10%.