Prominent among several factors that have been reported to affect the enantioselectivity of ketone reduction by washed suspensions of microorganisms are (i) the phase of batch culture at which the organisms are harvested and (ii) the concentration at which the ketone is supplied. Sulcatone (6-methylhept-5-en-2-one) is only poorly soluble in aqueous media, but in the present study a novel microbe friendly organic solvent mixture was employed to present this ketone in a range of concentrations to cell suspensions of four bacteria that were then incubated in an atmosphere of hydrogen for 6 h at 30 degrees C. The bacteria employed were Clostridium pasteurianum, Clostridium tyrobutyricum (two strains), and Lactobacillus brevis that had been harvested at various times from anaerobically growing batch cultures. L. brevis formed R(-)-sulcatol in high enantiomeric excess irrespective of its period of pregrowth in batch culture and the substrate sulcatone concentration (over the range 0.02-1.5 mM); however; in the case of all three clostridia, the enantiomeric selectivity of sulcatone reduction was substantially affected by both of these factors. It is likely that this reflects the possession by these clostridia of multiple ketone reductases differing in their K-m values for sulcatone and present in the organisms in differing proportions during the course of their batch culture growth.