The influence of pH on growth, and lactic acid and bacteriocin production by Lactococcus lactis subsp. lactis 140 NWC was studied during batch fermentation in a lactose-based complex medium. Growth and lactic acid production were modelled using a simple logistic equation while substrate consumption was found to be a function of growth and lactic acid production rate. The optimal pH for growth and lactic acid production was between 6.0 and 6.5. Bacteriocin production showed primary metabolite kinetics. pH had a dramatic effect on the production of the bacteriocin, lactococcin 140. A maximum activity of 15.4 x 10(6) AU (arbitrary units) 1(-1) was obtained after 7 h at pH 5.5. Maximum bacteriocin activity was achieved before the end of growth and was followed by a decrease in activity, which was due to adsorption to the cells of the producing organism, possibly followed by degradation by specific proteases. Both bacteriocin production and degradation rates were higher at pH 5.0 and 5.5, resulting in sharper activity peaks than at pH 6.0 or 6.5. On the basis of the experimental results a qualitative model for bacteriocin production is proposed.