Ammonia is produced during anaerobic digestion of protein-containing materials. While ammonia can be utilized by some members of the anaerobic population, excess ammonia can inhibit methanogenesis. High-solids anaerobic digestion may be especially sensitive to the effects of ammonia overproduction. This study explored the performance of a pilot scale high-solids anaerobic digestion process under various total ammonia nitrogen (TAN) concentrations. In general the high-solids digester could operate up to 1000 mg dm-3 without any inhibitory effect. The digester performed best when it was operated at TAN concentrations in the narrow range of 600 to 800 mg dm-3.