Adiabatic compressibility of red blood cells obtained from humans using a modified ultrasound velocity technique has been determined and correlated to the mean cellular hemoglobin concentration of the red blood cells. The ultrasound technique has an accuracy of better than 0.1%. The results show a mean adiabatic compressibility of 34.814×10−12±0.861 cm2/dyn for an average mean cellular hemoglobin concentration of 32.12±1.51 g%. The relationship between the MCHC and compressibility was investigated by suspending erythrocytes in saline of different NaCl concentrations. A correlation coefficient of 0.932 was obtained based on linear regression analysis.