A sensitive and specific assay has been developed and validated for the separation of a chiral sulfoxide drug candidate with simultaneous determination of the corresponding sulfide and sulfone in plasma by normal-phase LC-MS-MS. Separation was achieved on a Chiralpak AD (100 x 2.1 mm) column with a 2-propanol-hexane (80:20) mobile phase within 7 min. Aqueous mobile phase (2-propanol-10 mM ammonium acetate, 75:25) was added post-column prior to introduction into the heated nebulizer interface of a Sciex API 3 plus mass spectrometer, to avoid the explosion hazard of hexane-containing mobile phases in the presence of a corona discharge. The linear range of this assay was 5-2500 ng/ml. The accuracy and precision of the chiral sulfoxides, the sulfide and the sulfone were within +/- 15% across the linear range. The limit of quantitation for each component was 5 ng/ml based on the extraction of 0.25 ml plasma. The recovery for each component was between 82 and 120%. This assay was sufficiently sensitive and specific to support pre-clinical development studies in rats, dogs and monkeys.