Synergistic extraction system of lactic acid is examined to develop on in situ extractive fermentation process. The addition of tri-n-butyl phosphate(TBP) to the extraction system of lactic acid (HA) with tri-n-octylamine (TOA) diluted by hexane causes a large synergism. Extraction reaction with the mixed extractant is interpreted quite well based on the formation of mixed complex, HA . TOA . 2TBP. Though the addition of hexane solution containing TBP and TOA to the culture of Lactobacillus rhamnosus results in low lactate production, some lactate is produced.