I studied activation of serum creatine kinase (CK) by sulfhydryl reagents, and the optimum assay conditions. Dithiothreitol and β-mercaptoethanol were the best activators. Dithioerythritol, L-cysteine, glutathione, and mercaptoacetate were less effective. Over a dithiothreitol concentration range from 0.02 to 2 mmol/liter, enzyme activity increased with increasing dithiothreitol. Activation was maximum between 2 and 20 mmol/liter. At any dithiothreitol concentration the rate of activation was rapid, and the activity attained did not increase on prolonged preincubation. Thus, the extent of activation of serum CK by dithiothreitol is largely a thermodynamically-controlled, not a kinetically-controlled, phenomenon.