The self-diffusion coefficient of the extracellular hemoglobin of Lumbricus terrestris (mol wt 3.7 × 106 daltons) has been measured at protein concentrations ranging from 2 to 25 g/100 ml. The self-diffusion coefficient of human hemoglobin has been measured at concentrations between 10 and 43 g/100 ml. For these measurements, 14C-labeled hemoglobin was made to diffuse from one Millipore filter into three consecutively arranged Millipore filters containing unlabeled hemoglobin. After a suitable time the filters were separated, and the protein diffusion coefficient was determined from the distribution of radioactivity in the four filters with a table given by Kawalki (1894, Ann. Phys. Chem. 52:166-190.). The following results were obtained. The diffusion coefficient of Lumbricus hemoglobin is 1.2 × 10-7 cm2s-1 at a protein concentration of 2.1 g/100 ml, and is reduced to about 1/10 of this value when the concentration is 25 g/100 ml (T = 21°C). Between 0 and 16 g/100 ml the logarithm of the diffusion coefficient of Lumbricus hemoglobin falls linearly with concentration. Above 16 g/100 ml a marked increase in the concentration dependence of the diffusion coefficient is observed. Extrapolation of the data to zero hemoglobin concentration yields a limiting value of the diffusion coefficient of Lumbricus hemoglobin of 1.3 × 10-7 cm2s-1. The diffusion coefficient of human hemoglobin is 4.5 × 10-7 cm2s-1 at a hemoglobin concentration of 9.7 g/100 ml, and falls to 0.14 × 10-7 cm2s-1 at a hemoglobin concentration of 43.0 g/100 ml. In addition to diffusivities, the viscosities of human and Lumbricus hemoglobin solutions were measured in a wide range of protein concentrations.