A reversed-phase liquid chromatographic method was developed for 2-(4-isobutylphenyl)propionic acid (IBPP) in bulk drug and compressed tablets containing 100-600 mg of drug per tablet. The chromatographic system consists of a microparticulate octadecylsilica column and a mobile phase of aqueous acetonitrile buffered with chloroacetic acid. Whole tablets are disintegrated, and the IBPP dissolved in a solution of the internal standard in mobile phase. The resulting suspension is then centrifuged, and an aliquot of the supernatant chromatographed. This system separates IBPP from its analogues and degradation products in less than 10 min. Recovery of the drug from spiked placebo preparations was quantitative. Precision of the assay was 0.3 and 0.7%, relative standard deviation, for bulk drug and Motrin tablets, respectively.