A canine mammary tumor cell line established from a carcinoma of the solid type was used in this study. In 2 separate experiments, athymic nude mice were inoculated subcutaneously with 5 x 10(6) viable tumor cells. All tumor-bearing mice received subcutaneous injections thrice weekly of 2 micrograms/g body wt of selenium as sodium selenite or phosphate buffered saline. Control, non-tumor-bearing mice were injected with either selenium or buffer. Selenium administration did not significantly alter the growth of non-tumor-bearing mice. However, body weight of tumor-bearing mice was reduced approximately 10%. The average volume of tumors in selenium treated mice was reduced 76% in experiment I and 74% in experiment II.