A 10-year-old Laotian boy had homozygous deficiency of the third component of complement and recurrent bacterial infections beginning at age 5 months. Cellular and humoral immunity were normal, as were polymorphonuclear leukocyte chemotaxis and bactericidal activities. Serum complement-mediated hemolytic, chemotactic, and opsonic activities were deficient. In vitro addition of purified C3 to patient serum restored hemolytic complement to normal levels, and plasma infusion during each of four episodes of pneumonia significantly enhanced serum opsonic activity for as long as 36 hours. A renal biopsy specimen revealed mesangiopathic glomerulonephritis, although significant levels of circulating IgG immune complexes were not detected. These findings further support the association of C3 deficiency with immune-complex disease and suggest that plasma infusion may be an adjunct to antibiotic therapy in the management of severe pyogenic infections in patients with C3 deficiency.