White Leghorn chickens raised from 1 day old in an environment contaminated by the infectious bursal agent (IBA) had lower geometric mean titers (GMT), as measured by the hemagglutination-inhibition (HI) test to Newcastle disease virus (NDV) than control Leghorns reared in an uncontaminated enivronment. Immunosuppression, defined as a reduction in GMT, was most pronounced at 35-56 days old for Leghorns vaccinated with NDV at 1 and 28 days or at 28 days. In a separate trial with broilers, immunosuppression was similar at 42-56 days old. IBA infection in chickens also increased susceptibility to Marek''s disease (MD). The unvaccinated control chickens infected with IBA averaged 56.3% MD lesions whereas unvaccinated controls not exposed to IBA averaged only 18.1% macroscopic lesions. Of the HVT[turkey herpesvirus]-vaccinated chickens exposed to IBA, 20.7% had gross MD lesions whereas HVT-vaccinated chickens reared in an environment free of IBA had 2.99% gross MD lesions.