Male Syrian golden hamsters were fed a semisynthetic diet, given 12 weekly intratracheal instillation of 3 mg benzo(a)pyrene adherent to 3 mg Fe2O3, and then given either 100, 1600, or 2400 mu-g retinyl acetate (RA) per week intragastrically in 2 divided doses. One-half of the animals in each group were housed conventionally; the other half were housed in laminar flow units. Hepatic and serum vitamin A levels were markedly increased in hamsters given 1600 or 2400 mu-g RA per week compared to controls given 100 mu-g, which is adequate for growth and maintenance. In hamsters housed conventionally, increased RA intake was associated with an increased incidence of benign respiratory tract tumors. In all groups of hamster housed in laminar flow units there was a longer period to death with respiratory tract tumor than in conventionally housed hamsters; increased RA intake was associated with a somewhat lower incidence of respiratory tract tumors. Laminar flow housing significantly reduced the incidence of respiratory tract infection in non-tumor-bearing hamsters. Squamous papillomas of the forestomach were significantly reduced in all groups of hamsters given high levels of RA, regardless of housing.