In order to establish quantitative models of leukocyte functions, several morphometric parameters on individual white cells are needed. These include the diameter, volume, and membrane area of the cells and their nuclei in the undeformed state. A stereologic method was used to obtain these quantities from transmission electron microscopy of random sections through human white blood cells (neutrophils, lymphocytes, monocytes, and eosinophils). In order to estimate possible artifacts due to preparation of the cells for transmission electron microscopy, a detailed comparison with light microscopy was made. The results show that undeformed white cells in isotonic solution are spherical with many membrane foldings and have a significantly smaller diameter than that measured on blood smears. A method of chemical fixation was employed so that the shrinkage due to fixation of the cells is below the resolution of light microscopic measurements. Further, it was shown that all leukocytes, including lymphocytes, have much more membrane area than is needed to cover their volumes, and this membrane area remains constant when the cell is hypotonically swollen.