The mesangial matrix of the rat glomerulus was studied by transmission electron microscopy in specimens preserved by a modified technique, which avoids osmium tetroxide but uses tannic acid as a contrasting agent. It can be demonstrated that microfibrils are a major component of the normal glomerular mesangial matrix. They are non-branching tubular structures with a hollow centre, an undefined length and a thickness of approximately 15 nm. Microfibrils make up a dense fibrillar network interconnecting mesangial cells and glomerular basement membrane.