It is well established that cholera toxin (CT) produced by Vibrio cholerae acts as a potent mucosal adjuvant; however, the native form of this molecule causes severe diarrhea. Furthermore, both native CT and its B-subunit derivative bind to monosialogangliosides (GM1) in membrane raft microdomains on neural tissues and are thus unsuitable for use in humans. In this study, we evaluated the adjuvanticity of the CT A-subunit (CT-A) administered with ovalbumin (OVA) by the nasal route. We found that nasal administration of OVA plus CT-A elicited both mucosal and systemic antibody (Ab) responses. Immunization of mice with OVA plus CT-A resulted in the induction of OVA-specific IgA Abs in saliva and nasal secretions. Furthermore, significant OVA-specific serum immunoglobulin (Ig) G and IgA Ab responses were induced. Antibody-forming cell (AFC) analysis confirmed the Ab titer findings by revealing significant numbers of OVA-specific IgA AFCs in submandibular glands. In addition, splenic lymphocytes restimulated with OVA in vitro exhibited significant proliferative responses. Thus, CT-A might be a candidate for an effective adjuvant for inducing antigen (Ag)-specific Ab responses in human systemic and mucosal compartments, such as the oral cavity.