The three-dimensional architecture of filamentous components in the lamina densa of the Schwann cell basement membrane was studied in the mouse sciatic nerve by transmission and scanning electron microscopy after osmium-maceration treatment, and also by conventional electron microscopy. In conventionally prepared specimens, the lamina densa of the basement membrane was the most electron-dense, showing up as a felt-like layer 20-30 nm thickness. The interstitial surface of this layer had a spongy appearance with numerous shallow pits. Maceration of the specimens with 0.1% OsO4 for 2-4 days effectively removed amorphous, non-filamentous components from the basement membrane, thus exposing fine filamentous structures embedded in the lamina densa; these were about 10-15 nm in diameter and elaborately interwoven and/or connected with each other to form the framework of the lamina densa. Occasionally, some of them appeared to twine around adjoining collagen fibrils. The nature of these filamentous structures is discussed in terms of the chemical components of the basement membrane.