The rate of packing of the red blood cell column during centrifugation and the haematocrit have been found to depend on the cell flexibility. This has been altered by addition of formaldehyde, heating at 49, pH, removal of oxygen and aldosterone. Measurements of the haematocrit by dilution of haem oglobin and I131 labelled serum albumin solutions, added to these cell suspensions, did not show the same differences. An analysis of the data suggests that compression of the cells occurs during centrifugation, due to the weight of the packed cells pressing down on each other. Compared to haematocrit values for fresh whole blood of 45.5±1.5 per cent obtained by centrifuging, the true haematocrit is estimated to be at least 52.5±2.5 per cent.