Technical Abstract: Proventriculitis in broilers causes carcass condemnation when swollen proventriculi tear during evisceration. The cause of this proventriculitis is unknown but several infectious agents have been associated with it. One such agent, infectious bursal disease virus (IBDV), has been implicated as a cause of proventriculitis, but a direct effect of this virus on the proventriculus has not been proven. The role of IBDV in proventriculitis may be indirect due to its ability to cause immunosuppression. The objective of this study was to understand how immunosuppression affects the incidence of proventriculitis in broiler chickens. Immunosuppression was induced in commercial and SPF broiler chickens using chemicals (cyclophosphamide and cyclosporine) or virus (IBDV). All groups were then exposed to a proventricular homogenate produced from diseased birds. The incidence of proventriculitis in these groups was compared to that of controls.