After centrifugation of heparinised whole blood on Hypaque-Ficoll medium (s.g. 1.114, Ferrante and Thong, 1980) the specific gravities were measured of the layers of medium above and below the band of polymorphonuclear leukocytes (PMN), as well as of the PMN and sedimented erythrocytes. The erythrocytes and PMN were also examined for evidence of shrinkage. The medium above the PMN layer was found to be less dense than the medium below the PMN layer, which in turn was less dense than the original Hypaque-Ficoll medium. Examination of the cells showed that the sedimented erythrocytes and PMN were shrunken and of increased density. The results are consistent with a separation mechanism dependent upon osmotic loss of water from the sedimenting erythrocytes, forming a density gradient in the medium in which the PMN become buoyant.