Host defense in chickens against tumors caused by Marek disease virus is by cell-mediated immunity, which is inhibited by stress. In this study, metyrapone, an adrenal blocking agent, was administered to domestic chickens with Marek lymphoid tumors to determine whether cell-mediated response was increased in treated birds. Chickens naturally infected with Marek disease virus were fed a ration containing a narrow optimal dose of metyrapone (500 mg/kg of feed). Untreated control birds were fed the same ration without metyrapone. Visceral lymphoid tumors were absent or regressed in 82% (14/17) of chickens treated with metyrapone and in none (0/18) of the control birds. We conclude that metyrapone resulted in a reduced stress level in treated chickens that increased the effectiveness of T-cell defense against tumor growth.