Axenic cultures of Peridermium pini have been established on modified Shenk &'38 Hilderbrandt's and Harvey &'38 Grasham's media from naturally infected cortex tissues and aeciospore-infected calluses of P. sylvestris and from aeciospores collected from NE Scotland and East Anglia. The cultures occasionally produced immature smooth-surfaced, binucleate spores. Actively growing cultures infected P. sylvestris calluses but not seedlings or trees. In the experiment of fungal nutrition, (NH4)2SO4 appeared essential, sucrose, D-glucose, raffinose and D-sorbitol supported good growth while D-xylose, cellobinose and L-arabinose did not. Opt. medium pH proved to be 5.0-6.0. Axenic cultures were also obtained from 30-40&'37 of single sporelings of some East Anglia spore sources but not from NE Scotland sporelings. When inoculated at a high density, however, all spore sources from both East Anglia and NE Scotland readily formed colonies. Colonies from East Anglia spores mostly appeared smooth at the surface and distinct around margin while those from NE Scotland sources had fluffy surface and irregularly extended periphery. Rapidly expanding hyphal layers developed from both of the colony forms 3 months after inoculation. Callus tissue cultures of P. sylvestris, P. nigra var. maritima and P. mugo vars mughus, rostrata and pumilio were infected by inoculation with aeciospores from NE Scotland. Infections were characterized by formation of aerial hyphae on the callus surface and intercellular hyphae and typical haustoria in the callus tissue. Hyphae from some of the infected calluses penetrated the medium. Seedlings of the pines as above were infected at their cotyledon stage by inoculation with NE Scotland spores. Infections resulted in swelling, death of the seedlings and formation of spermogonia after a year and aecia after two years. Infections of young seedlings of 7 seed sources of P. sylvestris and the UK were examined 6 weeks after inoculation. Discolouration and necrosis of cotyledons were not always related to stem infection.