Hydroxy carboxylate viologen oxidoreductase (HVOR) contained in Proteus vulgaris cells was used in the reduction of chloropyruvic acid to D-(S)-chlorolactic acid. A synthetic electron mediator 1,1＇-carbamoylmethylviologen (CAV) was used to transfer electrons to HVOR. Three different methods for regenerating the mediator were evaluated : (I) electrochemically driven reduction; (2) reduction driven by formate using formate dehydrogenase; and (3) reduction driven by hydrogen gas using hydrogenase. On ii small scale, all three methods provided yields of D-(S)-chlorolactic acid of around 80%. For gram-scale conversions, the method using formate dehydrogenase and formate appeared best. It was found beneficial to add the chloropyruvic acid continuously during the reaction so as to avoid side reactions, Using 2.8 mg ml(-1) Of P. vulgaris cells, 4.4 mM CAV and a reaction time of 14 h, a preparative yield of 81% (6.6 mmol) of D-(S)-chlorolactic acid (ee > 97%) was obtained.