The capacity of tobacco infusion and cigarette smoke condensate to activate the complement pathway was investigated in vitro by hemolytic and immunological assays. Complement activity was studied using samples of normal human serum incubated at 37 degrees C with different dilutions of tobacco infusion and cigarette smoke condensate. In all mixtures containing tobacco infusion or cigarette smoke condensate at a concentration of 0.1: 1, total hemolytic complement showed an average progressive decrease of 90% and 50%, respectively, after 60 min of incubation. This decrease was associated with a decrease of both classical and alternative pathway activities. In addition, cleavage fragments of C3, C4, and factor B were identified with immunochemical analyses. This study suggests that tobacco infusion and cigarette smoke condensate are activators of complement either through the classical or alternative pathway.