PLATES LXllI-LXVTHE classical staining properties of amyloid, while satisfactory in most cases, may give equivocal results in some instances. The electronmicroscope (EM) demonstration that the basic structure of amyloid consists of fibrils 5-30 mp wide (Cohen and Calkins, 1959) opened a new method of investigation. Indeed, the fibrillar structure of all types of amyloid has been confirmed by many workers and has been further extended by Shirahama and Cohen (1965), who showed that the amyloid fibril consists of 1-8 laterally aggregated filaments, each about 7.5 mp wide, showing beading with a periodicity of 10 mp. At present the most reliable evidence of amyloid is the electron-microscope demonstration of fibrils.Amyloidosis is nowadays most commonly suspected or diagnosed during histological examination of biopsies (or necropsies) after formaldehyde fixation, routine processing and paraffin embedding. It would be an obvious advantage to be able to use the EM on such material. On the assumption that amyloid fibrils would not be affected by the processing, an attempt was made to examine different varieties of paraffin-embedded amyloid with the electron microscope. A case of polyarteritis was also examined which was suspected, but could not be shown by the conventional staining methods to have amyloidosis. Pepsin digestion was carried out on fresh and formaldehyde-fixed material in order to study the effect of digestion on amyloid fibrils. MATERIALS AND METHODS Tissue fixed in 4 per cent. formaldehyde in saline, processed in the usual way (a) Secondary arnyloid: a specimen of kidney with classical staining reactions and embedded in paraffin for up to 4 yr was selected as follows: with methyl violet, congo red and thioflavine-T.
