Benzo[a]pyrene (BaP), a well-known carcinogen, is frequently measured as an "indicator" of the potential dermal tumorigenicity of a sample. The present sequential high-performance liquid chromatography--high-performance liquid chromatography method overcomes problems in trace-level sample enrichment and recovery corrections encountered earlier. An amount of 5 g of naphtha or fuel oil is diluted to 10 ml with dichloromethane and spiked with a small quantity (ca. 0.25 micrograms) of 14C-labeled BaP tracer. A BaP-enriched fraction is obtained from a 1-ml aliquot of this sample by semipreparative chromatography on a Partisil PAC 10 column with dichloromethane-hexane (10:90) as the eluent, and concentrated to exactly 0.3, 0.5, or 1.0 ml in acetonitrile. Quantitation is performed using a reversed-phase Vydac 201 TP 5415 column with acetonitrile-water (75:25) as eluent and a Waters Model 420 E/420 AC fluorescence detector, employing an excitation/emission filter pair of 360/425 nm. The recovery of the radiolabeled tracer is evaluated by combustion of 50 microliter of the final isolate in pure oxygen, collection of the liberated 14CO2 in an alkaline desorber, and liquid scintillation counting. The recovery of BaP normally exceeded 90%, but values as low as ca. 50% were occasionally observed. Potential matrix interferences in the recovery determination were eliminated by sample combustion. The nominal precision of the overall method is approximately +/- 30% relative standard deviation at a BaP concentration of 30 ppb. The nominal analysis time for a single sample is approximately 4 h.