Enoate reductase (EC 1.3.1.31) from a Clostridium tyrobutyricum strain catalyses the stereospecific reduction of many different alpha, beta-unsaturated carboxylates, aldehydes and even some ketones. The enzyme accepts electrons from NADH and, 1.5 times faster, from reduced methyl viologen (1,1'-dimethyl-4,4'-bipyridinium). Another new type of non-pyridine nucleotide-dependent reductase has an extremely broad substrate specificity for 2-oxo-carboxylates and 2-oxo-dicarboxylates. In crude extracts from Proteus mirabilis and Proteus vulgaris, specific activities of 2-12 mumol product formed per mg protein per min can be found when reduced methyl or benzyl viologen is used as electron donor. The products are (2R)-hydroxy acids. Enoate reductase and 2-oxo-carboxylate reductase are suitable for electro-enzymic reductions in which catalytic amounts of viologens are continuously reduced in an electrochemical cell. This procedure has three advantages: (1) regeneration of NAD(P)H by a second enzyme and substrate is not required, (2) the unstable pyridine nucleotides are not required in the reaction mixture, and (3) the rate of the reaction can be observed continuously by measuring an electric current. Several yeasts, as well as aerobic and anaerobic bacteria, catalyse the reduction of NAD(P)+ by reduced methyl viologen. Such cells can be used for electro-microbial reductions when only pyridine nucleotide-dependent reductases are present. Information about the enzymes which catalyse the reduction of NAD(P)+ at the expense of reduced methyl viologen is given.