A gas chromatographic-mass spectrometric (GC-MS) method is described for the determination of the novel anticancer agent aphidicolin in plasma. The extraction was carried out in a solvent mixture of hexane:isopropanol (10:1) and the external standard aphidicolane was added after evaporation of the organic phase. The residue was then redissolved in a derivatizing mixture containing bis(trimethylsilyl)trifluoracetamide as sililating agent, pyridine, and trimethylchlorosilane, and allowed to react at 80 degrees C for 2 h. After GC separation of the derivatized samples, selected ion recording analysis was done, monitoring the ions at mass 523.3 and 448.3 for aphidicolin and aphidicolane, respectively. The mean recovery +/- SD of aphidicolin from plasma was 73.5 +/- 11.6% in the range from 5 to 800 ng. This method was applied to determine aphidicolin plasma levels in three cancer patients in Phase I clinical trials of aphidicolin-17-glycinate administered as a 1-h iv infusion. Two patients received dose of 290 mg/m2 and the third received 435 mg/m2. Aphidicolin plasma levels at the end of infusion were very low, and the drug rapidly disappeared from plasma with a terminal (beta) half-life of 2 h.