It has been established that the neutralization of lactic acid obtained during the lactose fermentation with the strain Lactobacillus casei can be accomplished by compatibility of the process with the enzyme hydrolysis of urea using immobilized urease. The kinetics of fermentation after the exponential phase of cell growth and the kinetics of urea degradation have been investigated. Mathematical models for both bioconversions have been derived. On the basis of these models, and algorithm for determination of experimental conditions which allow regulation of pH within narrow limits has been developed. It has been shown that the compatibility under conditions determined in this way results in a considerable enhancement of the lactose bioconversion as compared to the process with the addition of a neutralizing agent such as a 12% (v/v) water solution of ammonia. The multiple use of immobilized enzyme has been found to be limited.