By considering the blood as a mixture of ultrafiltrate and protein concentrate, the additive nature of compressibility and density from the components is utilized to deduce a linear relation between the compressibility and density for blood. This deduction also indicates that the intercept and slope of the linear relation are independent of the hematocrit, plasma protein concentration, and hemoglobin concentration of red blood cells. To verify experimentally this linear relation, saline and plasma dilutions on porcine or canine blood flowing in an extracorporeal circuit were carried out. The hematocrit of the experiments ranges from 0% to 55% and the plasma protein concentration ranges from 10 to 90 g/l. A resonance device in the circuit measured the density rhob of blood at 37 degrees C and an ultrasound system measured the sound velocity cb. The range of density is from 1,010 to 1,060 g/l and that of sound velocity is from 1,530 to 1,580 m/s. The linear relation that best fits the data of compressibility [computed as (rhob cb(2))-1] and density has a correlation coefficient of 0.9978. The linear relation is found to fit well the dependence of compressibility on density derived from the sound velocity data of human, horse, and porcine blood in the literature.