Simultaneous saccharification and fermentation of wood to ethanol has often been claimed to be less sensitive to infection than separate hydrolysis and fermentation. A previous study has shown that this was not the case. In the present study the influence of different parameters in SSF, such as cell concentration, nutrient composition and inclusion of the liquid from pretreatment, on the ethanol yield and lactic acid formation was investigated for steam-pretreated softwood. Commercial enzymes from Novo and Saccharomyces cerevisiae, in the form of compressed baker＇s yeast, were used for hydrolysis and fermentation. The formation of lactic acid was much higher when washed substrate instead of whole slurry from the pretreatment stage was used in SSF. The addition of extra nutrients, such as yeast extract, delayed lactic acid formation, whereas the addition of casamino acids stimulated lactic acid formation. The liquid from the pretreatment stage had a positive effect on the ethanol yield in SSF, as well as a reducing effect on the lactic acid formation. (C) 2000 Elsevier Science Inc. All rights reserved.