Lactococcus lactis has been identified as an efficient Gram-positive cell factory for the production of recombinant protein. The advantages of this system have led to the use of lactococci in several fields of biotechnology, e.g., the expression of bacterial and viral antigens for safe vaccination via mucosal immunization, the production of human cytokines and other therapeutic agents for in situ treatments, the use of lactococci as a cell factory for the production of specific compounds, and the pilot production of pharmaceutical products.