Three distinct o-diphenol O-methyltransferases (OMTs) were found in leaves of Nicotiana tabacum, variety Samsun NN. They could be clearly distinguished by differences in elution pattern upon chromatography on DEAE-cellulose and in specificity towards 16 diphenolic substrates. The phenylpropanoids caffeic acid and 5-hydroxyferulic acid, whose importance as lignin precursors is well known, were the best substrates of OMT I, but they were also efficiently methylated by the two other OMTs that showed a broader substrate specificity. The highest rates of methylation were observed by assaying these latter enzymes with catechol, homocatechol and protocatechuic aldehyde. The flavonoid quercetin, the major o-diphenol of tobacco leaves, was a good substrate for OMTs II and III, but was also methylated significantly by OMT I. The tobacco OMTs showed both para-and meta-directing activities with protocatechuic acid, protocatechuic aldehyde and esculetin as substrates. Para-O-methylation of the former substrate arose almost exclusively from OMT I whereas that of the two latter substrates from all three enzymes. In healthy leaves the total O-methylating activity varied very much with the batch of plants whereas the relative contributions of the three enzymes were rather constant. On an average, OMTs I, II and III acounted towards caffeic acid, respectively. In tobacco mosaic virus-infected leaves carrying local necrotic lesions we found the same three OMTs with the same substrate specificities, but with increased activities. The degree of stimulation of both OMTs II and III was 2-3 times greater than that of OMT I when the leaves had a moderate number of lesions, and 3-5 times greater with large number of lesions. It is very likely that the changes in both the pattern of the O-methylating enzymes and the concentrations of the naturally occuring o-diphenolic substrates are related to an increased biosynthesis of lignins and of lignin-like compounds. These aromatic polymers could be involved in the cell wall thickening associated with the hypersensitive reaction and with the resistance to virus spread that occur in the cells surrounding the local lesions.