The new computerized approaches for image analysis available for a few years permit, in an excellent and detailed way, monitoring cellular structural modifications during activation processes. Here, we presented morphometric data obtained on blood cells (platelets, monocytes, and PMN cells) after in vitro activation and under in vivo conditions related to atherosclerosis studied by means of electron microscopy. Area and perimeter form factors, nucleus to cytoplasma area ratio, and the surface density of platelet plasma membrane openings of SCCS presented significant differences compared with the respective significant differences compared with the respective controls. Ultrastructural markers of blood cell activation were revealed by the computerized morphometry applied on ultrastructural images.