Fenosan has been studied for its effect on the lipid peroxidation level of mitochondria and microsomes of the rat kidneys in different periods of nitrosodimethylamine-induced carcinogenesis as well as on the incidence of kidney tumour formation. A month after the treatment with the above carcinogen the lipid peroxidation in the rat kidney was activated. An additional injection of fenosan decreased the intensity of the lipid peroxidation and inhibited the tumour development in kidneys. It is supposed that anticarcinogenic action of fenosan is related to the membrane protection from the lesion by the carcinogen.