Cotinine is a sensitive and specific biochemical marker of exposure to cigarette smoke. We describe a simple solid-phase extraction of cotinine from plasma before quantification by HPLC. Extraction recovery was 97.9% +/- 11.0% for plasma concentrations of 5-400 micrograms/L. Baseline separation of cotinine and caffeine was achieved within 11 min of injection onto a C18 reversed-phase column. The mobile phase was citric acid/dibasic potassium phosphate (30 mmol/L each, pH 6.0) containing 100 mL of acetonitrile per liter. Within-day and day-to-day precision (CV) were 4.7% and 8.4%, respectively. We also describe a modification of the Nicotine Metabolite RIA kit (Diagnostic Products Corp.) for quantifying cotinine in plasma. Recovery of cotinine from supplemented plasma was within 10% of the expected value with this RIA kit. Interassay precision averaged 8.1% for samples in the range 50-400 micrograms/L; intra-assay precision averaged 3.6% at 230 micrograms/L and 8.7% at 53 micrograms/L. Correlation between the two methods was RIA = 1.13 HPLC + 14.8 (n = 128, r = 0.957, P less than 0.001). Both methods are technically simple to perform.