A chemically defined medium was prepared which adequately supported growth of a vaccine strain of Pasteurella tularensis. This medium differed from those previously described in: (i) concentration of components, (ii) a requirement for calcium pantothenate to obtain increased growth, and (iii) a low initial pH. Varying the concentration of individual components up to 10 times the standard amount did not increase the viable population or affect dissociation. The vaccine strain grown in this chemically defined medium, although lower in viable population, appears to retain its identity and to be equal in potency to that prepared by the conventional method. This preliminary study indicates the potential utility of this medium as a basis for controlled studies of a live bacterial vaccine in terms of growth characteristics, dissociation, virulence, and immunogenicity.