Four endochitinases (poly[1,4-(N-acetyl-beta-D-glucosaminide)] glycanohydrolase, EC 3.2.1.14) have been purified from leaves of Nicotiana tabacum cv. Samsun NN reacting hypersensitively to tobacco mosaic virus. Two of them are acidic proteins of molecular weights 27,500 and 28,500 and have been identified as 2 of the 10 pathogenesis-related proteins that are known to accumulate in tobacco in response to stress or pathogen attack. These two pathogenesis-related proteins, named "P" and "Q" when their biological function was unknown, account for one-third of tobacco mosaic virus-induced chitinase activity of tobacco leaves. They are serologically closely related to the two other chitinases, which can be considered as new basic pathogenesis-related proteins. These two basic chitinases exhibit higher molecular weights (32,000 and 34,000) and higher specific enzyme activity than the two acidic isoforms.