Enhanced anaphylatoxin and terminal C5b-9 complement complex formation in patients with the syndrome of hemolysis, elevated liver enzymes, and low platelet count. Complement activation was studied in ten consecutive pregnant women developing hemolysis, elevated liver enzymes, and low platelet count (HELLP syndrome) and ten other women with normal pregnancies. Blood samples for anaphylatoxin (C3a/C3a desArg and C5a/C5a desArg) and terminal C5b-9 complement complex determinations were drawn at delivery and 24 hours and 7 days later. Women developing HELLP syndrome had higher plasma levels of anaphylatoxins with delivery than did women with uneventful pregnancies. The plasma levels of terminal C5b-9 complement complexes at the time of delivery were increased as compared with levels 1 and 7 days after delivery in women with HELLP syndrome. The plasma concentrations of the anaphylatoxins and the terminal C5b-9 complement complexes returned to normal levels within 1 week after delivery in the HELLP group. The formation of C5b-9 complement complex indicates that the terminal part of the complement cascade has been activated and that C5a has been formed and eliminated. Complement activation with release of anaphylatoxins and terminal C5b-9 complement complexes may be one etiologic factor behind the elevated blood pressure, hemolysis, liver insufficiency, and platelet consumption seen in these patients. 