An improved postembedding method for electron microscopic localization of concanavalin A (Con A) receptors in ultrathin sections is reported. Materials are embedded in a hydrophilic resin, glycol methacrylate copolymerized with glutaraldehyde and urea, using a precisely controlled polymerization schedule that preserves ultrastructural integrity. Ferritin-Con A binds specifically in ultrathin sections of mouse spleen tissue and of Sephadex beads embedded in this resin. Quantitative investigations of procedures for decreasing nonspecific labeling demonstrate that preincubation of sections with bovine serum albumin reduces background to a very low level. This high-resolution method allows macromolecular affinity probes access to receptors in all cellular locations, while maintaining good morphological preservation.