In the cultivation of anaerobes in fluid media exposed to air it has been customary for some time to poise the medium at a favourable oxidation-reduction potential level by the addition of reducing agents or to retard diffusion of oxygen by the addition of agar to the medium. Allwin and Baldwin (1930) reviewed the use of reducing agents some ten years ago. Since that time, other agents have been introduced, particularly ascorbic acid (Klinger and Guggenheim, 1935) sodium thioglycollate and sodium formaldehyde sulfoxylate (Brewer, 1940). Spray (1936) reviewed earlier work on the use of agar and successfully used it in the cultivation of clostridia. Brewer (1940) combined the use of reducing agents and agar in a successful medium which has been widely used (McClung, 1940;Reed and Orr, 1941).These additions produce at least theoretical complications. To broth or peptone solutions, active oxidation-reduction systems, there is added another oxidation-reduction system in the form of a reducing agent; the establishment of equilibrium with the atmosphere is delayed by the addition of agar; a further oxidation-reduction system is introduced with the inoculum. At the same time we have no precise information on the limiting oxidation-reduction potentials for the growth of any species, though Quastel and Stephenson (1926), andFildes (1929) have probably approached these limits for two species as closely as the complexity of the system will permit.In the present paper an attempt is made to evaluate the influence of the various ingredients of the media on the O/R potential as measured at a polished platinum electrode and to correlate the potential with the minimum inoculum, of various species of Clo8tridium, to induce growth.