It is apparent that plant biomass provides an extensiverenewable resource ; however, its exploitation has beenlimited by the cost of hydrolysing plant structuralpolysaccharides to their component sugars. Althoughmicrobial cellulases and xylanases can be produced bysome organisms at very high levels, for exampleTrichoderma reesei produces 20 g of cellulase per litre ofculture (Mandels, 1985), the enzymes have extremely lowturnover rates when hydrolysing crystalline substrates.The specific activity of purified cellulase from T. reeseiis.0-6 U (mg protein)-’, which is 2-4 orders of magnitudelower than for most enzymes (Mandels, 1985). Furthermore, hydrolysis of the designated substrate isinvariably incomplete. Thus, the production of enzymeswhich exhibit a relatively high specific activity againstcellulose and xylan is crucial if the potential agriculturaland industrial benefits of plant biomass are to be fullyrealized. The development of such enzymes is not beyondthe scope of current technology, but will be possible onlywhen we have a detailed understanding of the structureand catalytic mechanism of existing cellulases andxylanases.