Chicks infected with Marek's disease (MD) virus (5000 plaque-forming units) at 1 day of age were given Amidapsone in the feed from 1 day of age until the experiment was terminated. Amidapsone, at all concentrations tested, reduced mortality due to Marek's disease. Chickens receiving Amidapsone 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%.