Serum plate (SP) and tube agglutination (TA) reactions and geometric mean Mycoplasma synoviae (Ms.) hemagglutination-inhibition (HI) titers were determined on chickens infected at 21 days of age with Ms. by foot pad, air sac and aerosol. One-half had been given Newcastle disease (ND) and infectious bronchitis (IB) vaccine in the water at 20 days of age, and the other half had not been vaccinated with ND and IB vaccine. Blood was taken at 3, 4, 6 and 8 weeks after Ms. infection. Geometric mean HI titers were determined on serums of chickens that had a foot pad injection of Ms. 8 weeks after Ms. infection and were bled at 1, 3 and 5 weeks after foot pad booster. Three weeks after aerosol exposure with Ms., the geometric mean HI titer was significantly higher in the ND- and IB-vaccinated birds than in the nonvaccinated birds. In the Ms. air sac-infected group of nonvaccinated birds the geometric mean HI titer had increased significantly 3 weeks after food pad challenge with Ms. In the Ms. air sac-infected group of vaccinated birds, the geometric mean HI titer had significantly increased 3 and 5 weeks after foot pad challenge with Ms.