We found that nisin catalyzed potassium efflux from glycolyzing Streptococcus bovis JB1 cell suspensions and that the steady state concentration of residual potassium was dependent upon the amount of nisin added. The relationship between nisin concentration and potassium depletion was a saturation function that had considerable cooperativity. By pre-incubating mixed ruminal bacteria with nisin and removing them prior to S. bovis JB1 addition, it was possible to estimate the ability of mixed ruminal bacteria to bind or degrade nisin. Low concentrations of mixed ruminal bacteria did not bind or degrade all of the nisin in 6 h, but little nisin remained if the mixed ruminal bacteria were present at more than 50 Wg protein ml 31 . Because cell-free ruminal fluid (10% v/v) inactivated the nisin in less than 2 h, and this inactivation could be counteracted by autoclaving, ultra-filtration or proteinase inhibitor, it appeared that there was an enzymatic degradation of nisin. Mixed ruminal bacteria degraded nisin rapidly, but this degradation did not prevent potassium depletion from mixed ruminal bacteria. These latter results indicated that nisin binding was faster than nisin degradation. The idea that nisin binding could protect nisin from degradation was supported by the observation that intact nisin could be extracted from mixed ruminal bacteria. These observations support the hypothesis that bacteriocins can be used to modify ruminal fermentation, but further work will be needed to see if these peptides can be produced economically.