Factors of stress induce important alterations in gene expression of plants. In tobacco new molecular species (absent in the healthy plants) appear in stress conditions; they may result from both gene derepression and posttranscriptional events. These molecules can be regarded as markers of stress. Two groups of markers are observed: non-specific, which are induced by various stresses of different origin (pathogens, abiotic factors), and specific, appearing as a response to a definite stress. It was established that common markers in tobacco leaves are the pathogenesis-related (PRs) proteins, one acidic peroxidase isoenzyme (REM 0.34), the phenylamides, the chlorogenoquinones and the callose. In tobacco seedlings common stress markers are three acidic peroxidase isoenzymes (REM 0.51, 0.60, 0.72), as well as the PRs proteins. The latter were shown to result from transcriptional activation of genes, whereas the other molecular species are likely due to posttranscriptional modifications. A specific marker of endoparasitic fungal ( e.g . P. tabacina ) pathogenesis in tobacco leaves and seedlings is the dramatic increase of ß-glucosidase activity; this response is not induced by other pathogens (ectoparasitic fungi, bacteria, viruses) and abiotic factors. Hence, the non-specific markers are informative for stress situation independently of the nature of stress factors, and can be suggested as components of a general defense system in tobacco. Specific markers may be used for diagnostic purposes.