A 54-kb plasmid (pND306) from Lactococcus lactis subsp. lactis 1252D encoded resistance to both Cu2+ and Sn2+. The copper resistance determinant was subcloned on a 12.8-kb Pvull DNA fragment and mapped using a number of restriction endonucleases. Six other copper resistant lactococcal strains were also identified and all contained multiple plasmids. Plasmids in five of these strains showed strong hybridization with a probe made using the 12.8-kb DNA fragment, however no chromosomal homologs were detected. The copper resistance determinant was further isolated as a 10.6-kb Sphl fragment and used to construct pND968 that expresses resistance to both copper and nisin.