Lactococcus lactis strain W8, which contains the nisin Z gene in its genome, grew well and produced nisin in cow's milk at temperatures of 30 to 37 degrees C. Maximum production of nisin was achieved at 6 h and was 4,000 activity units (AU) per ml in skim milk and 2,400 AU/ml in 3% fat milk. The organism produced nisin even in 20 times diluted skim milk and 3% fat milk at 1,000 and 600 AU/ml, respectively. Boiling of the fermented milk (pH 4.2) made with this culture allowed the separation of the liquid part (whey) from the curd. When 20 times diluted skim milk was fermented and the whey derived from it was lyophilized, the yield of nisin was 60,000 AU/g. The antimicrobial activity of the nisin preparation was stable for at least 1 year at refrigeration temperature. L. lactis W8 may have significant applications in the food industry for a cost-effective natural nisin preparation.