Aminoureidosulfone (AUS) was used as a feed additive to prevent mortality in Marek's disease (MD)-infected chickens in the laboratory. Chicks infected with MD virus (5000 plaque-forming units) at 1 day of age were given AUS in the feed from 1 day of age until the experiment was terminated. AUS, at all concentrations tested, reduced mortality due to Marek's disease. Chickens receiving AUS at the 0.002% level had a 4% mortality rate, those receiving 0.005% had 9% mortality, and those receiving 0.01% had 2% mortality; mortality rates in the respective controls were 44%, 49%, and 50%. However, lesion reduction in surviving chicks was minimal. When AUS was withdrawn from the feed, MD-induced mortality increased within two weeks; average body weight decreased also.