Cholera toxin (CD B subunit containing a trace amount of intact CT (cCTB) can serve as an adjuvant for inducing strong mucosal and systemic immune responses to antigens given intranasally (i.n.). To examine whether pure rCTB alone can enhance immune responses, mice were immunized i.n. with Aglffl of Streptococcus mutans alone, or mixed with or conjugated to rCTB. IgA antibody in saliva, nasal wash, trachea! wash, vaginal wash, gut wash, and IgG antibody in serum were analyzed. Both mucosal IgA in secretions and serum IgG antibody responses to Agl/n were enhanced by rCTB either mixed with or conjugated to the antigen. Protective immunity was examined in mice immunized i.n. with PspA from S.pneumoniae plus either cCTB or rCTB, and challenged either i.n. or intratracheally (i.t.) with virulent strains CA66 or WU2) of pneumococci. Mice immunized with PspA plus rCTB were protected against i.n. challenge but not against i.t. challenge whereas cCTB as adjuvant induced protection against both routes of challenge. rCTB can therefore serve as a mucosal adjuvant by the i.n. route for inducing antibodies that orotect aaainst mucosal infection.