Turnover of transferrin was measured in 62 mammals from 11 species (baboon, dog, goat, guinea pig, man, mouse, pig, rabbit, rat, rhesus monkey, and sheep) using iodinelabeled transferrin of homologous and/or heterologous origin. Protein turnover was determined either from the plasma protein-bound radioactivity curves or from the slopes of total body radiation. The volume of transferrin pool, expressed as equivalent milliliters of plasma, turned over per day correlated closely with species size (r = +0.977). Using the parabolic equation, y = axb, the constants for the relationship between body weight (x) and transferrin turnover (y) were a, 22.845; and b, 0.68. However, species of comparable weights with deviating transferrin turnovers do exist within the boundaries of this relationship. From a limited number of serum transferrin estimations in all species except for rhesus, six species appeared to have transferrin concentrations in a broad middle range (2.5-3.5 mg/ml). By comparison, dog and guinea pig had markedly lower, and pig and rat had markedly higher, serum transferrin concentrations.