The studies described in this thesis concern the examination of the equine mucosal immune system and the investigation of the efficacy of mucosal vaccination for protection of horses against the common respiratory pathogen, equine influenza virus (H3N8). Stimulation of immune responses at the nasophaymgeal induction site was measured by detection and enumeration of circulating antigen-speciflc antibody-secreting cells (ASC) in the peripheral blood. Cholera toxin B chain (CTB) given intranasally induced CTB-speciflc antibodies in both serum and nasal washes (mucosal effector site) which was preceded by the appearence of CTB-speciflc ASC in the peripheral blood. This confirmed the fiinction of a local immune system in the respiratory tract of the horse, and suggested the existance of a common mucosal immune system in horses. Techniques were developed to measure neutralising antibody in nasal secretions of ponies after infection with influenza virus (strain A/equi-2/Sussex 89) and intranasal vaccination with inactivated virus and CTB. The vaccination protocol, using systemically primed animals, elicited circulating influenza-specific ASCs, serum haemagglutination-inhibiting antibodies and nasopharyngeal virus neutralising antibodies. Intranasally vaccinated ponies were completely protected from challenge infection with influenza virus of the homologous strain. This has been the first demonstration of the efficacy of intranasal vaccination using inactivated influenza virus in horses.