The spin-lattice relaxation rates (1/T1) of serum, whole blood, and red cells were measured vs several concentrations of transition metal ions. For comparative purposes, the similar experiments were repeated in water. The rates show a linear dependence on concentration of each ion for water, but nearly a linear dependence for blood and its constituents. The influence of each ion on 1/T1 in a sample was expressed by the slope (relaxivity) of the least-squares fitting of 1/T1 vs ion concentration. The relaxivities of Mn(II) in serum and of Fe(III) in serum and blood are greater than those in water, whereas the relaxivities of these ions in the other cases and of all the other ions in call cases are smaller than those in water. However, the relaxivity data show that Cr(III) in serum and blood affects the 1/T1 rates. The ratio of relaxivity of each sample to that of water is known as proton relaxation enhancement (PRR) factor (epsilon). The epsilon factors for present data suggest that the added ions are bound to proteins, and only Mn(II) in serum and Fe(III) in blood and serum are accessible to water.