After aldehyde-fixation, treatment with phosphotungstic acid (PTA) in aqueous acidic medium was shown to produce an intense electron-opaque stain with minimal distortion of organelles. Mitochondrial matrix, cisternae of the endoplasmic reticulum, and the Z-band of muscle were densely stained, whereas membranes stood out in negative contrast. Staining of glycogen or lipid was not apparent. Under certain conditions the stain density reflected the concentration of protein based on the quantitative reaction of PTA with the positively charged groups, although the stoichiometry of the reaction between PTA and protein varied with the kind of protein. The staining conditions established should provide a base for the use of the method in quantitative electron microscopy, particularly on thin sections.

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