Lactococcus lactis subsp. lactis biovar diacetylactis DPC938 was identified as a bacteriocin-producing strain which exhibited a bacteriolytic effect on other lactococci. Lysis of such target strains was associated with decreases in optical density and release of the intracellular enzyme lactate dehydrogenase. DPC938 exhibits cross-immunity to L. lactis subsp. cremoris 9B4 (M.J. van Belkum, B.J. Hayema, A. Geis, J. Kok, and G. Venema, Appl. Environ. Microbiol. 55:1187-1191, 1989), a strain which produces the bacteriocins lactococcins A, B, and M. Genetic analyses revealed that a 15.5-kb region of DNA encoding these bacteriocins is highly conserved in 9B4, DPC938, and DPC3286, an overproducing derivative of DPC938. This region is located on a 72- and a 78-kb nonmobilizable plasmid in DPC938 and DPC3286, respectively. The bacteriolytic effect exhibited by DPC938 and DPC3286 on sensitive cultures is most probably due to the concerted action of all three bacteriocins. Since these cultures exhibit a lytic effect on lactococci, they have a potential application in the dairy industry as accelerators of starter lysis and hence accelerators of cheese ripening.