The topography and ultrastructure of the omentum in normal and stimulated mice were investigated with combined transmission (TEM) and scanning electron microscopy (SEM). The present study demonstrated that lymphocytes and monocytes were the principle cell types in the non-stimulated milky spot. Following stimulation with bacterial toxin and adjuvant there was an increased microvascular permeability to fluid, neutrophils, monocytes and fibrin deposits within the connective tissue matrix of milky spots, and a subsequent increased cellular migration across the mesothelial lining into the peritoneal cavity. Cellular migration from the milky spot to the peritoneal cavity is facilitated by the absence of a basal lamina from the submesothelial connective tissue layer, therefore, cells can migrate from the interstitial spaces of the milky spot to intercellular gaps between mesothelial cells without having to penetrate a fibrous barrier.