To obtain a possible correlation between cell density and cell size, the size of individual cells was measured under the microscope and their sedimentation velocity was measured; the density is obtained with Stoke's Law. Specifically, HeLa cells were sedimented in Joklik's medium at 30°C in a vertical glass tube with 2 mm×2 mm horizontal opening and cells observed with a horizontally aligned microscope. The overall mean density difference of HeLa cells at 30°C was 0.0316+−0.0044 g/cm3, resulting in a density of 1.0357 g/cm3 (with a density of 1.0040 g/cm3 for Joklik's medium at 30°C). Six size fractions had densities which were essentially the same within the errors of the mean densities of the fractions (from 0.0081 to 0.0202 g/cm3). The considerably varying deviations of individual densities from the mean suggested superimposed phenomena (see also Table I for microspheres of precise size). Careful observation in balancing countercurrent flow revealed microconvection over 5 to 15 μm regions, most likely based on small thermal differences in the horizontal plane.