It is shown that benzylselenocyanate in the diet significantly inhibits the incidence (percentage of animals with tumors) and multiplicity (number of tumors per animal) of adenocarcinomas in the colon and multiplicity of adenocarcinomas in the small intestine compared to those fed a control diet. Selenium-dependant glutathione peroxidase activity is significantly increased in kidneys and colon and small intestinal mucosae of animals fed a benzylselenocyanate diet compared to animals fed control diets. BSC in the diet is a superior inhibitor to both its sulfur analog BTC and the inorganic selenium (sodium selenite) during the initiation phase of carcinogenesis.