A single dose of vaccine for Mycoplasma bovis pneumonia, inactivated with saponin, was inoculated subcutaneously into 3-4 week-old calves. The calves were challenged 3 weeks later with a virulent strain of M. bovis on two occasions within 24h using the aerosol route. The calves were monitored for clinical signs and serological responses then post mortemed 3 weeks after challenge. The vaccine was shown to be highly immunogenic in calves and did not cause adverse effects. Vaccinated calves showed few clinical signs while all unvaccinated calves developed signs of pneumonia. There was a significant decrease in body weight gain in unvaccinated calves compared to vaccinates and a significant increase in lung lesions and rectal temperatures in unvaccinated calves. The vaccine also reduced the spread of M. bovis to internal organs. In conclusion the M. bovis vaccine produced a significant level of protection against a large virulent challenge.