An OH-form anion-exchange resin was used for the in situ product removal (ISPR) and pH control in the lactic acid production with immobilized Rhizopus oryzae in polyurethane foam cubes. The method was experimentally demonstrated both in shaking flasks and in an airlift bioreactor. In the experiments conducted, the broth pH was controlled at values higher than 6, and over 90% of the lactic acid produced by the fermentation was recovered by the anion exchanger. A mathematical model with absolutely unadjustable parameters was developed for the ISPR process. The theoretical calculations indicated that the model was in a good agreement with the experimental results. Consequently, the model was employed for computer simulations to analyze the behavior of the ISPR process. The simulation results showed that, the ISPR process could be performed in a wide range of operating conditions.