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Immunologic factors affecting viability of the Legionnaires' disease (LD) bacterium were studied in vitro and in vivo in mice and guinea pigs. In bactericidal tests, fresh human serum quickly killed LD cells. Heating fresh serum to 56°C for 30 min destroyed bactericidal activity; absorbing it with bentonite had little effect. Fresh normal human serum was more effective than guinea pig serum. Adding LD cells to fresh normal human serum caused a greater than 50% depletion in functional complement activity, apparently by activating the classic C-142 pathway, because human serum deficient in C4 was not bactericidal. Antibodies to the Knoxville 1 LD strain in guinea pigs showed enhanced complement-mediated bactericidal activity. Without complement, immune guinea pig or human sera prolonged in-vitro LD cell survival. Antibodies to Knoxville 1 in mice depressed in-vitro bactericidal activity of human complement against Knoxville 1. In-vitro bactericidal tests support in-vivo studies in subcutaneous chambers. Complement-deficient mice immunized with Knoxville 1 were (P < 0.01) less resistant to homologous challenge than nonimmunized mice. Immunized guinea pigs had a greater than 80-fold increase in resistance to subcutaneous-chamber infection.