4-[(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)-carbamoyl] benzoic acid (Am-80) is a novel retinobenzoic acid derivative possessing retinoid activity. Plasma concentration of this drug in clinical use is very low, less than 1 ng/ml, and could not be measured with direct immunoassays. A combination of high-performance liquid chromatography (HPLC) and radioimmunoassay (RIA) was developed for this drug in human plasma. Am-80 in 0.5 ml of human plasma was extracted by solid-phase extraction, and the extract was purified by reversed-phase HPLC. The immunoreactivity in the fraction of the eluate was measured by competitive RIA. The within- and between-assay variances were 4.1 to 15.5% and 4.1 to 7.0%, respectively. The limit of quantification was 0.04 ng/ml in human plasma, which was much lower than that of direct RIA, 0.6 ng/ml, previously reported. This assay system could be used to observe the pharmacokinetics of Am-80 in human even after dermal application at very low dose.