Density measurements were performed with an accuracy of 0.01 g/l by means of a density meter based on the "mechanical oscillator technique", on human whole blood, on plasma, and on erythrocyte concentrates. A highly significant correlation between blood density and hematocrit was found. If plasma and red cell densities are known, hematocrit can be estimated more precisely than using the centrifugation method. This fact is important if small variations of the hematocrit are to be determined. Such variations can be produced, e.g., by alteration of the transcapillary filtration equilibrium. Furthermore, the statistical distribution of plasma and erythrocyte densities allows to determine the range of variation of blood densities for certain hematocrit values and vice versa.