Effect of the air flow rate and concn. of the Saccharomyces cerevisiae yeast on the dissolved O concn. were studied during aeration in the bubble column and in the external-loop airlift reactor. Both active and inactive yeasts were used. The gas hold-up in the bubble column increased with the increasing air flow rate and the decreasing concn. of yeast both for the active and inactive yeast. The O concn. was higher in presence of the inactive yeast than the active one because of lower O consumption in the last case. In the active yeast suspension 1 g/dm(3) and air flow rate 0.0003 m/s, the O was quite totally used for the methabolic conversion. The vol. O transfer coeff. in the airlift reactor was lower than that in the bubble column.