A method is described for the estimation of mean cell volume of phagocytosing cells. The cells are coembedded with yeast particles of known size. By combination of data obtained from morphometrical analysis of sections in both the light and transmission electron microscopes, an estimate of the ratio between cell volume and yeast particle volume is obtained. The method makes use of the dissector but does not require measurements on serial sections or knowledge of section thickness. Evaluation of the method was done by studying the effect of phagocytosis of latex beads on macrophage cell volume and surface area. It was found that the surface area of phagocytosing macrophages remained constant although the cell volume increased by 27%. Furthermore, in phagocytosing macrophages, the amount of membrane enclosing intracellular vacuoles decreased by 28%, but this loss of membrane was balanced by the appearance of membrane enclosing the phagocytosed latex beads. The method described here may prove useful for morphometrical studies on phagocytosing leukocytes, as well as on intracellular parasites located within these cells.