The purpose of the present study was to determine the effect of dietary supplementation of selenite on experimental pulmonary metastasis of B16BL6 murine melanoma cells in C57BL/6 mice by means of an intravenous injection model. Three groups of mice were fed a basal AIN-93G diet containing 0.1 ppm selenium (control group) or the basal diet supplemented with 2 or 4 ppm selenium as selenite (experimental groups). Mice were fed the diet for two weeks before and after the intravenous injection of 0.75 x 10(5) viable tumor cells. At necropsy the number of tumors that developed in the lungs and their cross-sectional area were determined, and tumor volume was calculated. In the control group, 12 of the 15 mice had > or = 1 lung tumors. In contrast, only 4 of the 15 mice in each of the selenite-supplemented groups had > or = 11 tumors. The incidence of metastasis in mice fed the control and the 2- and 4-ppm selenium diets was 93%, 73%, and 53%, respectively. The median number of lung tumors was 53, 1, and 1 in mice fed the basal and the 2- and 4-ppm selenium diets, respectively. Tumor cross-sectional area and tumor volume were significantly decreased in selenite-supplemented groups. These results demonstrate that dietary supplementation of selenite reduced pulmonary metastasis of B16BL6 melanoma cells in C57BL/6 mice and also inhibited the growth of the metastatic tumors that developed in the lungs. It is concluded that selenite may be a useful adjuvant to prevent metastatic diseases in cancer patients.