A highly specific antisera was produced in New Zealand white rabbits against nisin Z, a 3400 Da bacteriocin produced by Lactococcus lactis ssp. lactis biovar. diacetylactis UL 719. A dot immunoblot assay was then developed to detect nisin Z in milk and whey. As few as 1·5 10−1 international units per ml (IU ml−1), corresponding to 0·003 μg ml−1 of pure nisin Z, were detected in carbonate-bicarbonate buffer within 6 h using chemiluminescence. When milk and whey samples were tested, approximately 0·155 μg ml−1 (7·9 IU ml−1) of nisin Z was detected. The detection limit obtained was lower than that of traditional methods including microtitration and agar diffusion.