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In cattle, the positive correlation between enhanced propionate production in the rumen and an increase in feed conversion after the treatment of low concentration of lasalocid or monensinhas been well documented1~4).Richardson et al5) showed that the increase in feed efficiency by monensin was a result of increased synthesis of propionic acid and decreased synthesis of acetic acid and n-butyric acid by micro flora in the rumen. Becuase monensin increases feed efficiency and reduces feed intake, its inclusion in animal feed results in a significant savings in feed costs to the cattle industry. Both monensin and lasalocid are being marketed as cattle growth performance enhancer under the trade name Rumensin and Bovatec, respectively.During our search for novel cattle growth performance enhancers from microbial sources using an in vitro rumen fermentation system2), a Streptomyces culture was found to produce a substance which elicited unusually high level of propionate production in rumen fluid. Production of the active substance wascarried out in a medium containing (in g/liter): tomato paste (Cantalina) 5.0, distiller' s soluble 5.0, meat peptone (type-SB, Marco Development Corp., Hackensack, NJ) 5.0, debittered dried 1.0, and K2HPO4 1.0. The medium pH was adjusted to 7.0 before sterilization. The

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