The addition to steaks of cell suspensions of a number of aerobic bacteria and of Saccharomyces cerevisiae greatly increased the rate of discoloration. Low inocula resulted in the more rapid appearance of the brown color of metmyoglobin, whereas high cell populations quickly produced the purple color of myoglobin. Sonically treated suspensions of Pseudomonas geniculata produced similar changes in surface color but less rapidly. No such effect was observed with Lactobacillus plantarum. The visible changes in color were found to be associated with the oxygen demand of the surface tissue including, of course, the demand of any contaminating microorganisms. Inhibitors of respiratory activity inhibited the rate of discoloration under normal atmospheric conditions. However, when the oxygen level in the atmosphere was reduced, the inhibitors had no significant effect. In an oxygen-free atmosphere, the steak surfaces were the purple color of myoglobin; at 10 mm oxygen pressure, the pigment was oxidized to metmyoglobin and the surface was brown in color. No bacterial activity was necessary for pigment oxidation under low oxygen pressures. Addition of dilute solutions of glucose oxidase resulted in rapid oxidation of the meat pigment to metmyoglobin both in extracts and on steak surfaces. More concentrated solutions resulted in further oxidation as evidenced by the appearance of a green color. Horseradish extract with a high peroxidase activity added with H202 resulted in rapid oxidation of the pigment but neither were very effective alone, although H202 did result in a browning reaction in aged steaks. It is concluded that the primary role of the bacteria in meat discoloration is in the reduction of the oxygen tension in the surface tissue. The implications of the data are discussed and a possible mechanism of myoglobin oxidation is proposed. I Journal article no. 2791, Michigan Agricultural Experiment Station.