During chemical pulping, lignin has to be removed from the cellulose. This process is toxic, energy consuming, and results in a low biomass utilization. For the pulp industry, it would be very beneficial to process trees which have either less lignin, or a modified lignin that is easier to separate from the cellulose. To obtain this goal, a basic knowledge of the biosynthesis of lignin is necessary. We have cloned several cDNAs encoding enzymes that play crucial roles in lignin biosynthesis in poplar, i.e. those encoding caffeic acid/5-hydroxyferulic acid-O-methyltransferase (COMT), caffeoyl-CoA-O-methyltransferase (CCoAOMT). cinnamoyl-CoA-reductase (CCR), cinnamyl alcohol dehydrogenase (CAD) and several peroxidases. By the antisense strategy, it is possible to unravel the in vivo role of these enzymes in regulating the amount and composition of lignin. Inhibition of the COMT activity in transgenic plants results in a dramatic change in the composition of lignin with the simultaneous appearance of a novel lignin unit and in a more difficult pulping. Inhibition of CAD activity in poplar results in an increased lignin extractibility during chemical pulping. Transgenic poplars with a modified CCR and CCoAOMT activity have been obtained as well as poplars modified in the activity of two lignin biosynthesis enzymes. In addition, the tissue-specific expression conferred by two different CCoAOMT promoters has been analysed in transgenic poplar.