TEM and three-dimensional models of the absorbing peripheral lymphatic vessel (ALPA) of the small intestine demonstrate that macrophages migrate from the interstitium into the lumen through the endothelial cell (transcellular pathway). Macrophages cross through a 'migration pore' which arises by an 'erosive process' in the cytoplasmic expansion of the endothelial cell. The pore is completely independent of the intercellular junction which is always sealed by junctional complexes. Furthermore several sites of contact between macrophage and endothelial cell membranes stress a possible functional role played by cellular interactions. Transcellular migration processes under normal conditions are compared with pathological or experimental ones.