The influence of carrier amounts of Fe, Ga, and Tm on the biodistribution of 67Ga-, 59Fe-, and 167Tm-citrate in mice was investigated. Our results suggest that 167Tm, unlike 67Ga and 59Fe, is not transported by transferrin in the blood. Of the three radioisotopes tested, 167Tm had the highest tumor/background ratio (10 h after the injection). The application of Fe carrier led to an enhancement of the elimination of 67Ga from the blood and muscles, resulting in a better tumor/background ratio.