Ultrastructural and cytochemical investigations of the interaction between Melampsora euphorbiae and its host Euphorbia peplus are described. Two types of collar around the haustorial neck could be recognized, corresponding to the maturity of the haustorium. Using various lectin-gold complexes as probes, different glycoconjugates were revealed in the fungus and host. Chitin was found in walls of uredinospores and intercellular hyphae but not of haustoria. d-glucose and/or d-mannose were strongly indicated in host starch grains and glycogen particles inside the intercellular hyphae, but only lightly in the fungal cytoplasm. Galactose residues and l-fucose were detected in fungal walls, more strongly in those of intercellular hyphae than haustoria. Galactose was also localized cytochemically in lomasome membranes of the fungus. β-Glucosides were detected in the fibrillar wall material bordering intercellular spaces of host tissue.