The aim of this study was to determine whether the biologically active complement peptides C3a and C5a are formed in pregnancy and whether amniotic fluid can activate complement. C3a and C5a are formed when complement is activated. They increase smooth-muscle contraction, vascular permeability, and histamine release from mast cells and basophils. Thirty pregnant women were studied, 16 with uncomplicated and 14 with preeclamptic pregnancies. The plasma C3a and C5a concentrations before delivery were significantly higher in the preeclamptic than in the normal group. The concentrations returned to normal within 1 week. Plasma, serum, and amniotic fluid from 12 pregnant women (eight uncomplicated and four preeclamptic pregnancies) were drawn in connection with delivery. Amniotic fluid was incubated in fresh autologous serum at 37C for 15 minutes. A dose-dependent formation of C3a and C5a was registered with increasing amounts of amniotic fluid.