Renal biopsies of seven patients with membranous glomerulonephritis were examined by light, electron, and immunofluorescence microscopy. All had characteristic changes of glomerular basement membranes, typically with bright granular membranous fluorescence of IgG and variable deposits of complement. Cellularity was normal or slightly increased due largely to mesangial cell proliferation. These cells assumed a variety of appearances referred to as resting, hyperactive, and dark. Resting forms occurred in cases with normal cellularity and limited complement deposition. Hyperactive cells showed ultrastructural evidence of increased secretory and digestive activity and were associated with proliferation and more pronounced complement deposition. Dark cells, interpreted as undergoing shrinkage and degeneration, were found in small numbers in all biopsies.