In the present study, T 1 and T 2 of phantoms containing serum sets with varying amounts of proteins, serum samples with certain amounts of proteins, serum diluted by distilled water, and serum treated with iron were measured. In addition, T 1 and T 2 of phantoms containing normal serum, diluted serum, and albumin-doped serum were also measured. Relaxation rates were plotted versus protein concentrations. The slope of relation was taken as relaxivity. The T 1 relaxivities of proteins were ranged from 0.035 to 0.080 s Ϫ1 (g/dl) Ϫ1 , whereas T 2 relaxivities were ranged from 0.24 to 0.68 s Ϫ1 (g/dl) Ϫ1 . The T 1 and T 2 relaxivities of transferrin iron were 2.40 and 2.60 mM Ϫ1 s Ϫ1 , respectively. The contributions of diamagnetic proteins and transferrin iron to the relaxation rate of serum were also calculated for each of diluted serum, normal and albumin-doped serum. The contributions and the average TP relaxivities(calculated by using individual relaxivities and the ratios of protein fractions in TP) were used for TP calculations. The agreement between the calculated TP and TP by autoanalyzer and also the agreement between average TP relaxivities and the TP relaxivities determined from dilution experiments show that the data of relaxivities are reliable. The results suggest that individual protein relaxivities explain the influence of serum TP composition on T 1 and T 2 relaxation times.