Lactococcus lactis subsp. lactis YBD11 was isolated from boza, a traditional fermented Turkish beverage. The bacteriocin produced by L. lactis subsp. lactis YBD11 inhibited the growth of Lactobacillus plantarum, Lactobacillus sake, Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, Micrococcus luteus, Listeria innocua, Enterococcus feacalis. Staphylococcus aureus, Staphylococcus carnosus, Pediococcus pentosaceus, and Bacillus cereus but nisin producer strain Lactococcus lactis SIK83 and Gram-negative bacteria were not inhibited Different enzyme, pH and heat treatments showed that the bacteriocin produced by L. lactis subsp. lactis YBD11 exhibited a similar behavior with control nisin. Maximum antimicrobial activity, 10240 arbitrary units (AU) ml(-1), was recorded in glucose-M17 broth after 18h at 30 degrees C. Based on the cell lysis treatments, bacteriocin of L. lactis subsp. lactis YBD11 was determined as acting bactericidal activity against Micrococcus luteus. Bacteriocin production occured throughout exponential phase, with the highest activity recorded at the end of this phase. Tricine-SDS-PAGE of partially purified bacteriocin gave the same molecular weight of 3.5 kDa as that of nisin. These results indicate that bacteriocin produced by L. lactis subsp. lactis YBD11 is a nisin-like bacteriocin.