Treatment of newborn mice with potent mouse interferon preparations resulted in an acute "early" syndrome characterized by inhibition of growth, delay in maturation of several organs, diffuse liver cell necrosis and death. When interferon treatment was discontinued at 1 week of life, mice appeared to recover, but subsequently developed a progressive glomerulonephritis ("late syndrome"). Treatment of newborn rats with potent rat interferon preparations also resulted in inhibition of growth, delay in maturation, and the subsequent development of glomerulonephritis. After infection at birth with lymphocyte choriomeningitis (LCM) virus, most strains of mice developed a similar acute early syndrome and surviving mice subsequently developed glomerulonephritis. We postulated that the endogenous interferon induced by LCM virus early in life was partially responsible for these syndromes. Administration of a potent anti-mouse interferon serum to LCM virus-infected mice neutralized the circulating endogenous interferon and inhibited the development of both the early and late syndromes. Our results suggest that large amounts of exogenous or endogenous interferon at a crucial stage of rapid growth or development of mice and rats can induce lesions in several different organs. Some lesions (i.e. the kidney) only become apparent weeks or even months after exposure to interferon.