The models describing the flow or the heat and mass transfer in stirred bioreactors can be easily adapted for the continuous phase from the stirred bed bioreactors. But, for the immobilized biocatalyst phase, new models have to be established. Therefore, in this paper the influences of the main factors on mixing for a bioreactor with stirred bed of S. cerevisiae immobilized cells in alginate have been analyzed.. The mixing efficiency continuously increases with rotation speed acceleration for the distances between the stirrers of 1.5d and 2d, the lowest mixing intensity being invariably recorded for 2d. The variation of mixing time obtained by

placing the impellers at a distance of 0.5d and d was sinusoidal and more evident for the closest positions of the stirrers. In all studied cases, the mixing time increases for about 3.5 - 4 times with the increase of the volumetric fraction of biocatalyst from 7 to 40%. The most efficient mixing has been obtained for biocatalyst particles with 4.6 mm diameter.