Aims: An approach to increase Streptococcus pneumoniae capsular polysaccharide (CPS) in the culture medium during fed-batch cultivation in bioreactor. Methods and Results: Streptococcus pneumoniae serotype 23F was cultivated in a 5-l bioreactor with nitrogen-sparging and followed by addition of air in the stationary phase. The amount of CPS released in the supernatant progressively increased under air sparging. The profile of cellular viability and optical density was similar in both cultures. Immunoelectron microscopy showed that the amount of tightly cell-bound CPS was higher in bacteria cultivated under nitrogen than under air. Conclusions: The stress caused by the addition of air at the stationary phase promoted a large increase of free CPS into the medium, as a consequence of the morphologic change in the capsule. Significance and Impact of the Study: The use of air in the stationary phase of the culture would greatly simplify the subsequent downstream process, allowing CPS purification from the supernatant. The direct consequence of this process improvement is the reduction of vaccine production costs.