In a field trial with chickens, the efficacy of killed Mycoplasma gallisepticum vaccine against respiratory mycoplasmosis was determined. Broiler chickens were vaccinated by aerosol administration at 3, 14, and 25 days of age, and observations were made of survival rate, body weight gain ratio, macroscopic lesion grade of airsacculitis, isolation of organisms, serologic test results, histopathologic changes, and clinical signs. Inhalation of the vaccinal aerosol did not influence livability and body weight gain ratios. In the control group, M gallisepticum, M synoviae, or both, wereisolated in 6 of 10 chickens at age 84 days and in 1 of 10 chickens at age 134 days. In all of the vaccinated groups (10 broilers each), however, results of bacteriologic cultural examination during the observation period were negative. The killed M gallisepticum vaccine also suppressed increase of antibody titers in the tube agglutination and the hemagglutination-inhibition tests at different intervals. Both the histopathologic changes and the clinical signs appeared minimal in the vaccinated group.