The influence of extensive accumulation of pus (n=20) or nonviable tissue (n=50) on complement has been studied. Plasma concentrations of C1 esterase inhibitor, C4, C3 and C5 were determined by electroimmunodiffusion technique. A good correlation between the extent of complement depletion and the amount of nonviable tissue or pus present was found. A prolonged state with reduced plasma concentrations of complement was in high frequency followed by the development of bacteremia. Patients with ongoing extensive complement activation were prone to develop ARDS. The relation between a depleted complement system and the development of bacteremia has been analysed and so has a relation between impaired oxygenation of arterial blood and products formed when complement is activated.