Infectious bronchitis vaccine virus is thought to cycle (i.e., pass from vaccine-virus-infected to susceptible chickens) in commercial broiler and pullet flocks. To simulate the effect of this cycling, mild infectious bronchitis vaccine virus was passaged in chickens serially six times. This sixth passage virus was used to infect chickens, which were then exposed to a moderately cold environment of 10 +/- 2 C and to Mycoplasma synoviae. In two separate experiments, the chicken-passaged vaccine virus resulted in a marked increase in the incidence of airsacculitis compared with nonpassaged vaccine virus. Intermediate passage levels also increased airsacculitis incidence but not as much as the sixth passage virus. In another experiment, virus chicken-passaged by contact transmission also caused increased airsacculitis incidence.