When mixed ruminal bacteria and alfalfa were incubated in vitro, monensin and nisin both inhibited methane production so long as the concentrations were greater than 1 mu M. Monensin- and nisin-dependent methane depressions caused a decrease in the acetate to propionate ratio (4.5 to 3.0). Total volatile fatty acid production was decreased by both monensin and nisin addition at concentrations greater than 2 mu M. Starch-digesting ruminal bacteria were initially inhibited by monensin and nisin, but this effect disappeared after two to four transfers, Nisin always inhibited cellulolytic bacteria, but the nisin-dependent inhibition of cellulose digestion was no greater than the inhibition caused by monensin. Monensin and nisin also inhibited amino acid degradation, and nisin was more effective than monensin in controlling the growth of Clostridium aminophilum, an obligate aminoacid-fermenting ruminal bacterium that can tolerate low concentrations of monensin. Because nisin was as potent as monensin, bacteriocins such as nisin may have potential as feed additives.