Many reports indicate that mixed microbial populations preferentially incorporate peptides or NH4+ rather than free amino acids [e.g. Pittman and Bryant, 1964]. As part of a study to evaluate a novel encapsulation process, an in vitro technique to examine amino acid degradation was developed using a compliment test based on the Reading Pressure Technique [Mauricio et al., 1999]. In this methodology, the ability of rumen microorganisms to ferment a pure carbohydrate substrate in a nitrogen-deficient medium was significantly correlated to the level of available nitrogen. It therefore followed that fermentation gas release could be used as a simple technique to examine and quantify nitrogen availability. The study described compared three nitrogen sources offered either alone or in combination.