The pressure–flow relations of single erythrocytes and of heparinized whole blood flowing through glass tubes 4–15 μm in diameter have been studied. Driving pressures ranging from 0 to 8 cm H2O were used to produce mean velocities of flow of 0–1000 μm∙s−1. The velocities were measured by high-speed cinemicrophotography. The velocities of latex microspheres suspended in water and in plasma were also measured. By comparing the mean flow velocities of erythrocytes with that of microspheres in water, values for the apparent relative viscosity were obtained.Results show that the pressure–velocity relationship in each case is linear. The relative viscosity of plasma ranged from 1.50 to 2.25 which agrees with values obtained by other methods of viscometry. A single erythrocyte flowing through the tubes increases the flow resistance and the apparent viscosity by about 30% over that of the suspending medium alone. The percentage increase is the same using plasma or Ringer solution as the suspending fluid. The apparent viscosity of whole blood even at hematocrits of over 80% is only about 55% higher than the viscosity of plasma.