aspergillus fumigatus was cultured in disc-turbine-agitated vessels and in an air-lift fermentor. in the agitated vessels the yield of cellulase was reduced when the agitation rate was increased, although extracellular protein levels rose. the enzyme complex itself was shown to be exceptionally stable under conditions similar to those in the agitated vessels; so probably shear damage to the mycelium had occurred, liberating intracellular contents. these appeared to contain an inhibitor that could be removed by fabricated inorganic protein absorbents, such as kieselguhr and alumina however, the inhibitor was not likely to be protease, since only relatively low levels could be detected and its identity has not been established. the use of an air-lift fermentor avoided the shear effects due to use of the disc turbine agitator in the conventional fermentors, and yields of enzyme were then found to increase by about 20, maximum yields being obtained at maximum k//la values. 25 refs