One or more factors which occur in bovine rumen fluid stimulate the growth of a strain of Butyrivibrio. The stimulating material is heat stable, organic in nature and non-dialyzable. It cannot be extracted from rumen fluid with lipid solvents and is retained in part on anion and cation exchange resins. It can be eluted from the resins with strong acid. It is stable to enzymatic hydrolysis by trypsin. Granular mucin or bovine saliva will partially replace the stimulatory activity. The part of the material which was not replaced by mucin did not appear to be any compound that is commonly used to stimulate bacterial growth. The presence of a possible inhibitor for the growth of a strain of Butyrivibrio was demonstrated. / Master of Science