Abstract: Ultramicrotomes conventionally use a glass fragment, or fragment of similar material, as a knife. Such knives have the disadvantage of an uneven back surface inherent in the production of such a knife by fragmentation. Using the complementary fragment and mounting the same slightly spaced from the knife back, provides an anti-curl device. A knife holder, having a groove in which the knife fragment is mounted and a pivoted member with a matching slot to hold the anti-curl device, provides positioning of the anti-curl device and provides access to the specimen when the anti-curl device is pivoted out of proximity of the cutting edge.