The objective of this study was to investigate the effects of nisin and propolis on in vitro fermentation of a 60:40 forage:concentrate diet using the rumen simulation technique and to compare their effects with the antibiotic monensin (positive control). Nisin, propolis ethanol extract and monensin were added daily at 2 mg, 100 mu L (contained active substances of 2 mg crude propolis) and 5 mg to the fermentation vessels, respectively. Monensin caused expected changes in fermentation patterns (a significant decrease in NH(3)-N concentration, acetate and butyrate production, protozoa counts and a significant increase in propionate production). Nisin and propolis did not cause beneficial effects on fermentation efficiency except NH(3)-N concentration. NH(3)-N concentration was reduced (p<0.05) by them. In conclusion, the results of this study indicate that nisin and propolis might be useful additives to decrease ruminal ammonia production and to improve nitrogen utilization by ruminants.