Cholera toxin (CT) is a strong systemic and mucosal adjuvant that greatly enhances IgG and IgA immune responses, but its adjuvant effects for cellular immunity, particularly class I-restricted cytotoxic T lymphocyte (CTL) responses, are less well understood. In the present report, CT and the purified non-toxic B component (CTB) were assessed for their ability to facilitate class I-restricted CTL induction to soluble proteins as well as to permit sensitization of target cells for CTL-mediated lysis. Priming for ovalbumin (OVA)-specific CTL occurred following oral exposure to a combination of OVA with CT plus CTB. In addition, CTB mixed with soluble proteins and administered intravenously primed mice for antigen-specific class I-restricted CTL. Target cells could also be sensitized for CTL-mediated killing following their exposure to soluble antigen and CTB in vitro. These results indicate that combinations of CT and CTB not only enhance antibody responses, but also have an immunomodulating effect to allow sensitization and priming for antigen-specific class I-restricted CTL.