Therapeutical plasma exchange can be carried out by using blood cell separators based upon centrifugation or in equipments with membranes. We investigated whether activation of the complement system took place in polyvinylchloride tubes used in the blood cell separator. We also examined the changes in the classical and alternative complement pathways by an analysis of functional haemolytic titers, as well as the levels of C4, C3D, C4, albumin and alpha 1-antitrypsin. No measurable activation of the complement system was found. The decreased levels of the C3, C3d, C4, albumin and alpha 1-antitrypsin in the sera after PE could be a consequence of the haemodilution.