Subcellular localization of the pathogenesis-related PR-1 proteins of unknown function was studied in roots of resistant Nicotiana tabacum cv Xanthi nc uninfected or infected in vitro by the black root rot fungus Chalara elegans , using polyclonal or monoclonal antibodies raised against PR-1a (or b1) protein. In healthy tobacco roots, the PR-1 proteins were found to be present in low amounts in intercellular space material, over cell walls and over secondary thickenings of xylem vessels. All these cell compartments were significantly enriched in the PR-1 proteins in infected tobacco root tissues. Cell wall outgrowths, typically induced in hypodermal cells by Chalara elegans infection and wall appositions formed in cortical parenchymal cells in response to infection, were also the sites of PR-1 protein accumulation. In contrast, very little occurred in electron-translucent intercellular spaces. PR-1 proteins were also detected in the wall of both inter- and intracellular fungal hyphae invading root tissues, but not in axenically cultured Chalara elegans or hyphae developing outside roots. These results are discussed in relation to the possible role of the PR-1 proteins in defence mechanisms in roots.