SUMMARYPathogenesis-related bl (PR-b~) protein and other serologically related PR-b proteins, known to be associated with both tobacco mosaic virus infection and chemical treatments, such as with polyacrylic acid (PAA), were detected by immunofluorescence microscopy around epidermal, palisade and lacunar leaf ceils in hypersensitive tobacco plants (PAA + line). After gentle fixation and embedding in Lowicryl K4M, PR-b proteins revealed by immunogold labelling were located in the cytoplasm and apoplast. After fixation and embedding in Epon, the PR-b proteins were found to accumulate mainly in the intercellular spaces. These observations support results obtained from biochemical tests.