Several processes have been developed for the enzymic conversion of lignocellulose to fermentable sugars. Most of these processes have employed the cellulolytic enzyme system from Trichoderma reesei. The action of a commercial cellulase preparation, Celluclast, from Trichoderma reesei on microcrystalline cellulose is compared with that of cellulolytic enzymes from other microorganisms. It is concluded that the Trichoderma system is not unique. More effective enzyme complexes can be produced from other microorganisms. In particular, systems from different Aspergillus species are shown to be more effective. The enzymic conversion of lignocellulose to fermentable sugar has not yet been scaled up, undoubtedly for technical and economic reasons. The cellulose substrate is very inaccessible, and furthermore the enzymic hydrolysis involves several consecutive reactions, each of which may be rate limiting. Enzyme costs have a great impact on process economy. The conversion of cellulose into fermentable sugar requires approximately 100 times more enzyme protein than the corresponding hydrolysis of gelatinized starch. Therefore, with today's enzyme technology, the use of cellulose as a raw material is not competitive with processes based on starch.