Three-dimensional measurements of eleven kinds of cells, obtained from serial sections of five different organs, excised from eleven adult mammals of different body sizes-from a 40 g mouse to a 450 kg cow-were made. In order to minimize technical errors all organs were submitted to standardized fixation and staining procedures. Twenty cell diameters (at the nuclear level) were measured in each of the 7 microns serial tissue section which were made in two planes, after a 90 degree rotation of the fixed and embedded organ specimens. The mean values of the cell diameter measurements were submitted to a cluster analysis by means of a computer program, to establish the cell type groups with similar morphometric characteristics. The dendrograms of the cell-type groupings were then compared with the results obtained by applying the traditional statistical analysis of the cell sizes (in micrometers) in the three dimensions of space, and also with the principal component analysis. With the three statistical methods we came to analogous conclusions. The empirical allometric exponents for the three cell diameters, when expressed independently as functions of body mass, are not significantly different from zero, and in consequence cell sizes are independent of body mass. The physiological meaning of the body-size-independence of the mean three cell diameters is discussed.