SUMMARYGlass knives for ultramicrotomy were considerably improved by coating the cutting edge with a film of evaporated tungsten metal. Knives treated by this method gave up to ten times as many acceptable sections as uncoated glass knives. They could also cut thinner sections and harder tissues than ordinary glass knives, and eliminated some of the cutting artefacts produced by them. No explanation for this improvement was found although several possibilities were examined.