Bentonite particles can be sensitized by both somatic O and H (flagellar) soluble antigens. After fixation, washing will remove the unadsorbed O antigen, and also partially elute the flagellar antigen.The serological response of rabbits to immunization with typhoid vaccines was compared by Widal bacterial agglutination and by bentonite flocculation. Bentonite had a limited ability to react with early type antibody, but matched the sensitivity of Widal aggultination for secondary response agglutinins. While bentonite particles are preferentially clumped by secondary response antisera, both mercaptoethanol resistant (7S) and mercaptoethanol-sensitive (19S) antibodies are responsible for bentonite flocculation.