1. Rates of reduction of a number of azo food dyes were measured in anaerobic cultures of Proteus vulgaris. The rates of colour loss were found to be zero order under conditions in which the concentration of viable cells remained constant. 2. A significant increase in the rate of reduction followed the onset of cell mortality. 3. The zero-order rates correlate with the redox potentials of the dyes. A mechanism consistent with these observations involes an extracellular non-enzymic reducing agent which acts as an electron shuttle between dye and cellular reducing enzymes.