Subcellular distributions of (103)Ru in tumors and livers from Ehrlich's solid tumor-bearing mice (given (103)Ru-chloride intravenously) and in AH-130 ascites hepatoma cells (incubatedin vitro with (103)Ru-chloride) were investigated by differential centrifugation. Fractionations showed that most of the total radioactivity was recovered in the mitochondrial, and cell dibrisplus nuclear, fractions. The highest relative radioactivity per protein was found in the mitochondria1 fraction. These results indicate that (103)Ru accumulates especially in the mitochondria. (97)Ru and (67)Ga in livers showed similar subcellular distributions.