Lactococcus lactis subsp. lactis 484 produced a proteinaceous antibacterial substance designated as 'lactococcin' capable of inhibiting members of the Lactococcus group, Bacillus cereus, Staphylococcus aureus, and Salmonella typhi. Growth of this culture in the presence of 2-30-mu-g/ml of ethidium bromide or acriflavin or novobiocin, and at elevated temperatures (39-degrees and 41-degrees-C), could not produce any lactococcin-negative (Lap-) variants. However, protoplast-induced curing with lysozyme was successful in developing Lap derivatives. Two types of cured derivatives, namely Lac- Lap+ and Lac- Lap-, were obtained. Lap- variants were also lacking sucrose-fermenting ability (Suc+) and lactococcin resistance (Lap(r). The lactose-negative (Lac-) variants and Lap+ were clearly lacking the largest (65 Md) plasmid. However, Lap- Suc- Lap(s) variants lost a 2 Md plasmid. L. lactis subsp. lactis 484 transferred lactose-fermenting ability as well as Lap+ Suc+ Lap(r) phenotypes simultaneously to L. lactis subsp. lactis LM 2306 and LM 0230 by surface mating at a frequency of 10(-4) and 10(-1) per donor respectively. However, cured Lac- Lap- transconjugants could not transfer Lac+ Lap+ Suc+ Lap(r) phenotypes to any of these recipient strains. Our results indicate that Lac+ and Lap+ Suc+ Lap(r) phenotypes are associated with 65 Md and 2 Md plasmids respectively. Conjugal transfer of 2 Md plasmid is possible only in the presence of a conjugative 65 Md plasmid.