1. An antibactericidal factor in the serum of patients with urinary-tract infection could be partially purified by absorbing the serum with the homologous organism and treating the absorption complex with a weak acid buffer.
2. Material obtained in this way contained mainly immunoglobulin G.
3. Lipopolysaccharide extracted from the homologous organism was as effective as whole bacterial cells in removing the factor from patient's serum.