In normal human epidermis, lanthanum penetrated into the basal lamina-basal cell interspaces, including half-desmosome areas, non-specific intercellular junctions, desmosomes, close approximations and most of the occluding zonules and macules. The majority of what were previously claimed to be tight junctions were permeated by lanthanum, revealing that they were actually gap junctions. Maculae occludentes which follow desmosomes, for instance, were found not to be tight. On the other hand, the interspace between the stratum granulosum and stratum corneum was only rarely permeated. Lanthanum filled the intercellular channels between the lateral plasma membranes of granular cells but was stopped near the exit of the channel into the interspace. Truly tight junctions (zonulae occludentes or the distal junctions of the stratum granulosum) were found to be responsible for this occlusion. Intercellular spaces between the horny cells were not commonly permeated by lanthanum.