A liquid chromatography/electrospray ionization mass spectrometry (LC/ESIMS) method for the determination of 4-(3-ethoxy-2-hydroxypropoxy) acrylanilide (M-1), the active metabolite of suplatast tosilate, in human plasma was established. Plasma samples were extracted with diethyl ether, separated on a C(18) column with a mobile phase of acetonitrile-10 mm ammonium acetate solution containing 0.1% formic acid (28:72, v/v) and detected by ESIMS. The method was linear over the concentration range 0.15-60.0 ng/mL. The lowest limit of quantification was 0.15 ng/mL. The intra- and inter-run relative standard deviations obtained from three validation runs were all less than 8.6%, and the intra- and inter-run relative errors were all less than 3.1%. The method was successfully applied for the evaluation of pharmacokinetic profiles of M-1 in healthy volunteers.