Complement system has been investigated in the first 15 days postburn. Complement titer (measured by a kinetic method of immune hemolysis) decreases immediately after thermal injury. This depression is mainly explained by mechanical leakage and is related to burn surface. No anticomplementary effect has ever been found in this period. After 48 hours complement titer rises. Magnitude and rate of rise are inversely related to the severity of burn trauma (depth and surface). Infection occurrence is statistically related to but somewhat independent of complement levels and interferes with the rate of increase of complement levels. The basis of the rise to normal or elevated complement levels remains unexplained.