Sixty-four lactococcal strains isolated from natural whey starters were screened for the presence of the nisin structural gene by polymerase chain reaction. Seven of them showed a specific PCR product of 320 bp; only two produced antagonistic activity and were resistant to nisin. Southern blots of SmaI-digested DNA from PCR-positive strains hybridized with a nisA probe displayed a location of the gene on different SmaI fragments. Among PCR-positive strains, nisin producers showed specific transcript after reverse transcriptase-PCR, as well as some non-nisin-producing strains. The RT-PCR product could not be shown in one non-nisin-producing PCR-positive strain.