The extraction of lactic acid from aqueous solution by a number of extractants dissolved in different organic diluents was explored over a wide range of pH values. Tri-n-octylmethylammonium chloride (TOMAC or QCl), dissolved in oleyl alcohol, butyl acetate, and a mixture of hexane and oleyl alcohol, was found to be most appropriate in terms of high extraction capacity and simplicity of operation. Biocompatibility between the extraction reagents and lactic-acid-producing microbes was also examined. The results indicated that TOMAC was highly toxic, and both butyl acetate and hexane exhibited some detrimental effects. However, oleyl alcohol exhibited no toxicity and could, therefore, be considered the most suitable diluent. During extractions with TOMAC concentrations no greater than 0.1 kmol.m(-3), implementation of a cleaning column filled with oleyl alcohol seemed to effectively remove trace amounts of TOMAC dissolved in the fermented broth; extractive fermentation of lactic acid was accomplished, despite a little decline in lactic acid production relative to production obtained for a control experiment. However, higher TOMAC concentrations are desired for better extraction of accumulated lactic acid to reduce its own inhibitory effect. A cleaning column packed with the cation-exchange resin Amberlite IR-120B was more effective. As a result, satisfactory extractive fermentation was achieved, thereby proving the feasibility of efficiently producing lactic acid by extractive fermentation, with TOMAC dissolved in oleyl alcohol as the optimum extraction system.