The growth of ten species of ectomycorrhizal fungi was measured in liquid media containing different organic and inorganic nitrogen sources. All fungi grew well on ammonium. Growth on nitrate was generally lower, although there was considerable variation between different isolates of the same species. Suillus variegatus, Piloderma croceum, Paxillus involutus, Hebeloma crustuliniforme and unidentified pink and white isolates often grew as well on organic nitrogen sources as on ammonium. Growth of the other species was more variable. Isolates of Thelephora terrestris and Lactarius rufus varied in their ability to use bovine serum albumen (BSA) but two Laccaria species were poor at using the protein as a nitrogen source. The ability of mycorrhizal and non-mycorrhizal Pinus contorta Dougl. ex Loud, plants to utilize BSA was also examined. Non-mycorrhizal plants and mycorrhizal plants infected with either T. terrestris or the unidentified pink ectomycorrhizal symbiont were supplied either with ammonium or with BSA. Growth of plants supplied with BSA was significantly increased by mycorrhizal infection with the pink symbiont and not significantly different from that of plants supplied with ammonium, but non-mycorrhizal plants were unable to use the protein as a nitrogen source and had significantly lower yields and nitrogen contents than infected plants. In contrast, mycorrhizal infection with T. terrestris had no effect on growth or nitrogen contents of plants supplied with protein. The results are discussed in relation to possible physiological differences between ectomycorrhizal fungi occurring at different successional stages of forest development.