A systematic analysis using serial sectioning of the round window membrane (RWM) in the cynomolgus monkey was performed. Light and transmission electron microscopy (LM and TEM) revealed that the RWM rim may be endowed with gland-like structures with glyco-protein material secernated into the window niche. This was detected in one third of the specimens. The secreted material displayed waste material and scavenger cells. There was also a rich network of capillaries, lymph channels, and sinusoidal veins containing leukocytes. Their abluminal surfaces displayed mature plasma cells and monocytes. These findings suggest that in certain primates the middle ear may have developed specific immunoprotective means for disposal of foreign and noxious substances before they reach the inner ear.