The primary object of this experiment was to evaluate the potential role of the antioxidants, selenium and vitamin E, in the anti-tumour defence of mice internally irradiated with 90Sr. Comparison in terms of neoplastic response was made between mice kept on a selenium and vitamin E deficient diet and mice given the same deficient diet but administered selenium and/or vitamin E in a controlled manner. The influence of simultaneous oestrogen treatment, known to promote radiogenic osteosarcoma induction, was also investigated. Non-irradiated mice were used as controls. Results are presented as crude and actuarial tumour incidence. No significant difference in tumour yield or actuarial tumour incidence was found when the differently treated mouse groups were compared, and accordingly no support was gained for the theory that the antioxidants selenium and vitamin E constitute a critical part of the complex defence system against neoplasms.