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A 57-year-old Japanese female was admitted because of edema, hypoproteinemia and proteinuria. Her histopathological findings of renal biopsy specimen were quite unique. Light microscopic findings suggested membranous glomerulonephritis, but no significant deposition of immunoglobulins or complements was detected in glomeruli by immunofluorescence. Electron microscopic examination revealed irregular thickening of the glomerular basement membrane (GBM). The GBM had no electron-dense deposits, but numerous microparticles varying in shape and size were present in all the thickened GBM and occasionally in the mesangium. The microparticles were round or oval in shape, and the size varied widely, measuring 25-290 nm (mostly 40-120 nm). The cytoplasmic infolding into the GBM by podocytes was seen. The large-sized particles had microgranules, mimicking free ribosomes seen in podocytes or endothelial cells. We conclude that cytoplasmic infolding and subsequent degradation may, partly, contribute to the formation of microparticles in the GBM.