Lactococcus lactis LAC48, a spontaneous erythromycin resistant mutant strain of N8, showed an increased nisin resistance from 1000 to 5000 IU ml(-1). The cells of LAC48 produced 10 times more nisin than the cells of the N8 strain. The results of Northern and Western analyses of LAC48 suggested that the majority of the exceptionally high level of nisin resistance in strain LAC48 was not due to changes in the expression level of the natural immunity genes. Strain LAC48 represents a good candidate to be used in industrial fermentation to achieve a high nisin titre.