Aims: The aim of this study was to modulate the lactococcal proteolytic system for enhancement of the cheese ripening process. Methods and Results: The genes encoding PepN, PepC, PepX and PepI peptidases of a highly proteolytic Lactobacillus helveticus strain were transferred into Lactococcus lactis in a food-grade cloning system. A comparison of the relative peptidase activities from the transformants with those from the untransformed host, determined in the conditions of maturing cheese, showed that an increase in peptidase activity could be achieved by introducing a selected peptidase gene from Lact. helveticus into L. lactis. Conclusions: Recombinant L. lactis starter strains, carrying a peptidase gene from Lact. helveticus, may have an important contribution to the proteolysis of maturing cheese by producing an additional peptidolytic enzyme activity. Significance and Impact of the Study: The results will be of importance in shortening the ripening period and production of special cheeses (e.g. reduced-fat cheeses) with improved characteristics.