An additive method has been developed to obtain nearly optically pure (S)-ketoprofen wS.-2-(3-benzoyphenyl)propionicacid, ＞96% enantiomeric excess, eex by using lipase OFfrom Candida rugosa.for the first time. The effect of surfactantsselected as additives.on the enantioselective hydrolysis of 2-chloroethyl ester of ketoprofen by the crude and the purified C. rugosa lipase (lipase OF) was investigated. Except for Tween-60, Tween-80 and nonyl phenol polyethyleneoxy etherOP-10., most of the other surfactants tested had inhibitory influence on the lipase. As for the enantioselectivity, only Tween-80 displayed a positive effect. Moreover, the concentration of Tween-80 was found to be a sensitive factor affecting the activity and the enantioselectivity, when either the crude or the purified enzyme was used. Upon addition of the two emulsifiers at their optimal concentrations, i.e., 2%(w/v) Tween-80 and 3%(w/v) OP-10, the crude enzyme activity was greatly enhanced up to 13 and 15 times, respectively. On the other hand, the enantiomeric ratio (E value) increased from 1.2 to 6.7 for the crude lipase and from 8 to＞100 for the purified lipase in the presence of 8% or 2%(w/v) Tween-80.