The routes by which Mycoplasma gallisepticum initiates infection during outbreaks of conjunctivitis in house finches remain uncertain. As M. gallisepticum recovered from the cloaca of chickens remains viable for up to 3 days in chicken faeces, the possibility of spread via faecal contamination has been suggested. To test the hypothesis that food or water contaminated with M. gallisepticum may initiate infection, 20 house finches were experimentally inoculated by the oral or the conjunctival route. Clinical and immunological responses were compared. All inoculated birds seroconverted, thus demonstrating infection. Only two of the birds inoculated via the oral route developed very mild unilateral conjunctivitis while all 10 of those infected by eye-drop inoculation developed severe bilateral conjunctivitis. The orally inoculated birds had reduced levels of activity for only a few days, while those infected by conjunctival inoculation had reduced activity for several weeks. M. gallisepticum DNA was detected in conjunctival swabs by polymerase chain reaction in only three orally inoculated birds but in all birds in the conjunctivally inoculated group. Antibodies developed more slowly after oral inoculation than after conjunctival inoculation. We showed that oral exposure to M. gallisepticum can initiate infection, disease, and a serological response, which suggests that food or water contaminated with secretions or excretions may be a route of transmission between house finches.