Whereas protein engineering of enzymes and structural proteins nowadays is an established research tool for studying structure-function relationships of polypeptides and for improving their properties, the engineering of posttranslationally modified peptides, such as the lantibiotics, is just coming of age. The engineering of lantibiotics is less straightforward than that of unmodified proteins, since expression systems should be developed not only for the structural genes but also for the genes encoding the biosynthetic enzymes, immunity protein and regulatory proteins. Moreover, correct posttranslational modification of specific residues could in many cases be a prerequisite for production and secretion of the active lantibiotic, which limits the number of successful mutations one can apply. This paper describes the development of expression systems for the structural lantibiotic genes for nisin A, nisin Z, gallidermin, epidermin and Pep5, and gives examples of recently produced site-directed mutants of these lantibiotics. Characterization of the mutants yielded valuable information on biosynthetic requirements for production. Moreover, regions in the lantibiotics were identified that are of crucial importance for antimicrobial activity. Eventually, this knowledge will lead to the rational design of lantibiotics optimally suited for fighting specific undesirable microorganisms. The mutants are of additional value for studies directed towards the elucidation of the mode of action of lantibiotics.