The modifying effect of selenium as sodium selenite on chemically induced respiratory tract tumours was tested in Syrian golden hamsters. Groups of 40 hamsters per sex (controls 60 per sex) were fed the following semisynthetic diets: control diet (0.1 p.p.m. Se, low fat); high-Se diet (5 p.p.m.); high-fat diet (20% sunflower oil); or high-Se + high-fat diet. After an adaptation period of 30 days on the diets, respiratory tract tumours were induced by intratracheal instillation of benzo[a]pyrene attached to ferric oxide. The experimental period was 429 days for males and 374 days for females. Respiratory tract tumours included mainly epidermoid papillomas, epidermoid carcinomas and combined epidermoid and adenocarcinomas. Selenium included either in a low-fat or high-fat diet did not influence the tumour response in the respiratory tract or in other organs. Neither was there a correlation between the serum or liver Se levels in the presence of respiratory tract tumours. The tumour response in the respiratory tract and also in other organs was slightly respiratory tract and also in other organs was slightly enhanced by dietary fat.