Heat-stable and heat-labile factors of normal human serum involved in opsonophagocytosis and intracellular killing of Pseudomonas aeruginosa by human polymorphonuclear leukocytes were investigated. Specific natural IgG antibodies, properdin, and a heat-labile factor (presumably C3 proactivator) were all required for phagocytosis and bactericidal activity. This finding suggested that an alternate pathway of the complement system was activated. Addition of immune IgG antibodies to the system eliminated the requirement for properdin and the heat-labile factor. Thus, two different mechanisms of complement activation can participate in the opsonophagocytosis of P. aeruginosa.