Daizo, Ushiba (Keio University School of Medicine, Tokyo, Japan), Taiji Nakae, Takehisa Akiyama, and Yoshio Kishimoto. Characterization of “clearance” factor and “cell-bound” antibody in experimental typhoid. J. Bacteriol. 91:1705–1712. 1966.—The “clearance” factor produced in the serum of rabbits immunized with a killed vaccine of an S-type virulent strain of Salmonella enteritidis was identified as O agglutinin and characterized immunochemically as a class of macroglobulin with a sedimentation coefficient of 12.7S. The macroglobulin, which was isolated by means of diethylaminoethyl cellulose column chromatography and sucrose density gradient centrifugation, showed a single protein peak when subjected to cellulose acetate electrophoresis and analytical ultracentrifugation. Its activity as agglutinin and “clearance” factor was inactivated both by mercaptoethanol treatment and absorbtion with the specific O antigen. The H agglutinin was also isolated as a single globulin fraction with relatively low molecular weight, and was characterized as a 7S γ-globulin. This fraction had no “clearance” activity. A macroglobulin, similar to that identified in the rabbit serum, was extracted with urea from the cells of mice immunized with the S-type killed vaccine, but not from mice immunized with a live vaccine (R-type mutant), although this latter vaccine is very effective in producing a solid immunity against typhoid disease. The relation of this “cell-bound” antibody to the mechanism of “cellular” immunity in experimental typhoid is discussed.