The proteolytic system of Lactococcus has been investigated in detail due to its pivotal role in allowing growth in milk and the development of flavour and texture in cheese. For a recent com-prehensive review the reader is directed to Kunji et al. (1996). In summary, casein is degraded by a membrane-anchored serine proteinase (PrtP) with many of the resulting oligopeptides being sufficiently small to allow them to be transported into the cell via an oligopeptide transport system (Opp), where they are further processed by a variety of intracellular peptidases. Amino acid, and di- and tri-peptide transport systems also exist but there is only poor growth in milk when mutants are deficient in PrtP (Kunji et al., 1995). Commercial culture adjuncts (mesophilic and thermophilic starter cultures) are available to promote proteolysis in cheese and can aid in the development of a consistent cheese flavour (Fox et al., 1996; Beresford and Cogan, 1997). It is notable that over-expression of a lactococcal proteinase did not result in an acceleration of cheese ripening or in an enhancement of flavour (Law et al.,1993). Mutants lacking combinations of up to five