Mortality was 60% when chickens without detectable maternal antibody to avian adenoviruses were inoculated intra-abdominally with 10(6) plaque-forming units of AMG 5(2a), a type-8 avian adenovirus. Other results were macroscopic and microscopic lesions in a wide range of organs, statistically significant depression of body weights, AMG 5(2a) virus in the liver and feces, and high virus-neutralizing antibody titers to AMG 5(2a). The disease produced was similar to that described in a previous report of AMG 5(2a) infection of chickens, and similar to inclusion body hepatitis as described in the literature. In contrast, similar inoculation of chickens with maternal antibody to type-8 avian adenovirus resulted in no mortality, lesions in the liver only, no depression of body weight, AMG 5(2a) virus in the feces only, and relatively low virus-neutralizing antibody titers. During this study a hemorrhagic-aplastic anemia syndrome occurred in both AMG 5(2a)-inoculated and control chickens in one trial. Pathologic, virologic, and serologic findings indicated that the spontaneously occurring disease was not caused by an avian adenovirus.