The effects of temperature, ration constituents, antibiotics and manure age on the ultimate methane yield (Bo, litre CH4/g volatile solids fed (VSf)) were investigated using 4-litre batch fermenters. The average Bo for fermenters maintained at 30-60 degrees Celcius (at 5 degrees Celcius intervals) was 0.328 litre CH4/g VSf. The Bo at 65 degrees Celcius averaged 0.118 litre CH4g VSf, but this low yield was attributed to unstable fermentation rather than decreased substrate availability at that temperature. These results agreed well with Bo values estimated from daily-fed fermenters. Chlortetracycline and monensin did not affect Bo; however, monensin did delay the start of active fermentation in batch fermenters. The average Bo of manure from cattle fed 91.5, 40 and 7% corn silage were 0.173, 0.232 and 0.290 litre CH4/g VSf, respectively. The average Bo for 6-8 week old manure from a dirt feedlot was 0.210 litre CH4/g VSf. (Refs. 14).