Direct evidence was obtained to indicate that the magnitude of the antibody response of mice to bacterial (Leuconostoc) dextran B1355 is influenced in a negative and positive manner by the action of suppressor T cells (Ts) and amplifier T cells (TA), respectively. Thus, the antibody response to this well-defined antigen can be added to the growing list of other microbial polysaccharide antigens, e.g., Type Ill pneumococcal polysaccharide (SSS-III), Neisseria meningitidis group A and C capsular polysaccharides, Pseudomonas aeruginosa lipopolysaccharide, Streptococcus mutans polysaccharide, and Serratia marcesans lipopolysaccharide, whose antibody responses are known to be influenced by regulatory T cells (Ts and TA). Evidence was obtained to indicate that populations of these regulatory T cells expand in a clonal manner in response to immune B cells and that such expansion is facilitated by IL-2.