The mechanism of killing of 12 serum-sensitive strains of mucoid Pseudomonas aeruginosa isolated from patients with cystic fibrosis was investigated. A quantitative assay indicated that more than 90% of cells were killed in 50% normal human serum (NHS). All strains failed to grow in NHS concentrations of greater than 10%. Killing was unaffected by adsorption of NHS with the mucoid bacteria or chelation with MgCl2-ethyleneglycol bis(beta-aminoethyl ether)N,N1-tetraacetate (MgCl2-EGTA) but was abolished in serum heated to 50 C for 20 min. Incubation of NHS with mucoid P. aeruginosa reduced the hemolytic capability of MgCl2-EGTA-chelated NHS against rabbit red blood cells by 56%-99%. Killing of the serum-sensitive mucoid strains was also seen in hypogammaglobulinemic serum. These data suggest that killing of such strains by NHS can occur via antibody-independent activation of the alternative pathway of complement. The importance of this finding lies in the implication that complement levels in the lungs of patients with cystic fibrosis who are colonized by these organisms are inadequate to deal with this chronic, progressive infection.