1. The serum of two patients with infections of the upper urinary tract contained a specific factor which inhibited its bactericidal action against the Gram-negative organism causing the infection.
2. The serum of the patients was fractionated by gel filtration and anion-exchange chromatography and pooled fractions were examined for antibactericidal activity.
3. Fractions showing antibactericidal activity appeared to contain only immunoglobulin G and it is suggested that bacterium-specific antibactericidal immunoglobulin G competes with immunoglobulin M, the major bactericidal antibody, for sites on the bacterial surface.