The serum of a 23-yr-old woman with prolonged disseminated gonococcal infection syndrome failed to normally promote hemolysis of sensitized sheep red blood cells (RBC). The patient's serum was deficient in the eight component of complement (C8) as determined by functional assays, immunoelectrophoresis, and quantitative immunoprecipitation. Functional titers of each of her other complement components were normal. No serum inhibitors of C8 were detected. The patient's serum supported activation of both the classical and alternate complement pathways. Her fresh serum lacked any bactericidal activity against Neisseria gonorrhoeae, but addition of purified C8 or complement donor serum restored bactericidal activity as well as RBC hemolytic activity. Her serum gave normal opsonization of yeast particles and staphylococci and had normal capacity to coat sensitized RBC with C8 and C4 and to generate chemotactic activity. No defects were observed in the patient's blood coagulation mechanisms. Complement-mediated bacterial lysis may be important in human defense against bacteremic Neisseria infections.