The effect of cross-synergism in enzymatic hydrolysis of ball-milled Avicell, alkali-treated straw cellulose (ATSC), cotton and filter paper was investigated using mixtures of Fusarium oxysporum and Neurospora crassa enzymes. The experimental data were fitted according to an empirical hyperbolic model which utilized two parameters, the maximum conversion (x(max)) and the enzymatic hydrolysis time corresponding to 50% of x(max) (t(1/2)). The model can predict conversion of polysaccharides as a function of hydrolysis time. Both model parameters were found to be strongly dependent on the crystallinity index as well as on the degree of delignification of the substrate. Up to 60% cellulose hydrolysis can be achieved when the crystallinity index of Avicell is reduced from 94.8% to 63.3%. The percentage increase of x(max) due to delignification was higher than the corresponding increase of t(1/2). The extent of cross-synergism depends strongly on crystallinity index and degree of delignification. This type of synergism has been found to be significant in the case of substrates which are resistant to hydrolysis, such as Avicell (with high crystallinity index) or cotton. Cross-synergistic phenomena caused by enzymatic mixtures can double cellulose hydrolysis yield with delignified straw as compared to the hydrolysis yields achieved by single-microorganism cellulases. (C) 1996 Elsevier Science Ltd.