The cellulase, Spezyme CP from Genencor, widely used for the simultaneous saccharification and fermentation (SSF) of cellulose to ethanol, contained substances inhibitory to the growth of Klebsiella oxytoca P2, emphasising the need to check for inhibition effects in SSF experimentation. Also, the preparation contained enough beta-glucosidase activity to prevent cellobiose accumulation in SSF with a conventional non-cellobiose fermenting yeast: this finding is relevant to attempts to evaluate novel recombinant cellobiose-fermenting microbial strains.