Summary Infection of protocorms of Dactylorhiza purpurella and a Cymbidium hybrid by an endophytic mycorrhizal fungus (Rhizoctonia sp.) led to a marked stimulation of respiration. Peak oxygen uptake often coincided with formation and digestion of pelotons. Treatment of protocorms with culture filtrates or mycelial extracts of the mycorrhizal fungus also enhanced oxygen uptake. Activities of polyphenol oxidase, ascorbic acid oxidase, peroxidase and catalase were greatly increased in infected protocorms. Peak activity often coincided with peak oxygen uptake and digestion of pelotons, except for peroxidase where peak activity occurred earlier. The orchid/endophyte system is compared with host/pathogen interactions in terms of the possible relationship between changes in enzyme activity and defence mechanisms.