The use as an embedding resin for ultrathin sectioning of a cross-linked triple copolymer of N-vinylpyrrolidone, acrylonitrile, and ethylene glycol dimethacrylate is described. The first of these components is miscible with water, in all proportions, and can be used as a dehydrating agent, or, alternatively, ethanol may be used in the standard way. Polymerization is carried out at 37 degrees C or even lower temperatures. This resin is unsuitable for use after osmium fixatives, but after permanganates it gives results similar to epoxy. Photographs of rye root-tip cells fixed in permanganate and sectioned in this resin are presented. Because of the water-permeable nature of the product and low polymerization temperature, this resin appears to have possibilities for histochemistry.