The synergistic role of infectious bronchitis virus (IBV) and mixed infections of Mycoplasma synoviae (MS) and Escherichia coli (EC) in the airsacculitis syndrome was evaluated in gnotobiotic chickens. Relative air sac lesion score indexes, in descending order of severity, from various combinations of organisms were: 9.5-IBV, MS, EC; 6.8-IBV, EC; 4.5-IBV, MS; 2.7-IBV; and 0.5-MS, EC. Infectious bronchitis virus caused a mild fibrinous inflammation. M. synoviae combined with IBV increased heterophilic and follicular lymphoid infiltration and mortality. E. coli combined with IBV increased exudation and prolonged airsacculitis. Concentrations of fibrinogen, gamma globulin, and total plasma proteins were elevated significantly by combined infections of IBV, MS, and EC (P < 0.01).