The asymmetric reduction of ethyl 3-oxobutanoate to ethyl (S)-3-hydroxybutanoate by baker's yeast (Saccharomyces cerevisiae) immobilized in calcium alginate was studied. The reaction was carried out under aerobic conditions with glucose fed as electron donor. Using immobilized cells at a concentration of 6 g dw biomass per liter of reaction volume and with mean particle diameter d(P.mean) = 1.2 mm resulted in the same specific reduction rate as in the reduction using freely suspended cells. The enantiomeric excess of ethyl (S)-3-hydroxybutanoate was greater than or equal to98%. At larger particle sizes, the specific reduction rate decreased while the enantiomeric excess of ethyl (S)-3-hydroxybutanoate remained high at greater than or equal to98%. At d(P.mean) = 1.2 mm but using higher cell concentrations (22-37 g dw l(-1)), the average specific reduction rate was reduced by 22% compared to the specific reduction rate using low cell concentrations or using free cells. Under these conditions, the enantiomeric excess of ethyl (S)-3-hydroxybutanoate decreased to 83-90%. Calculation of the concentration profiles in alginate beads showed that among the reacting species, severe diffusion limitation is expected only for oxygen. This may have led to the lower reduction rates of the immobilized cells at larger particle sizes and at high cell concentrations and in addition, to the change in enantioselectivity.