A novel radiometric high-performance liquid chromatographic (HPLC) method was developed for the determination of [14C]bucromarone in human plasma. The procedure involved the addition of non-radiolabeled bucromarone hydrochloride to each plasma sample as an internal standard; the plasma sample was then extracted, and the bucromarone was separated from its metabolites and endogenous compounds by reversed phase HPLC. The concentration of [14C]bucromarone in each plasma sample was calculated from the ratio of the amount of radioactivity in the eluate fraction corresponding to bucromarone and the peak height of the ultraviolet absorbance (210 nm) of the non-radiolabeled bucromarone used as an internal standard. The lower limit of quantitation for bucromarone free base in this assay was 8 ng/ml when [14C]bucromarone succinate had a specific activity of 0.5 microCi/mg. The coefficients of variation for the experimentally determined concentrations of bucromarone in spiked plasma samples were 6.8 and 14.3% at concentrations of 80 and 20 ng/ml, respectively. This method was used to determine concentrations of bucromarone in the plasma of healthy volunteers who were given intravenous infusions of [14C]bucromarone succinate. In general, the methodology should be applicable to any radiolabeled compound that possesses appreciable ultraviolet absorbance.