Aniline is widely used as an intermediate in the synthesis of dyes. It is also used in the manufacture of pharmaceuticals, photographic developers, shoe polish, etc. Exposure to aniline is toxic because it produces methemoglobin. In humans, blood methemoglobin levels are often measured as an index of exposure to aniline. Here a method is described for identification and quantification of aniline by gas chromatography-mass spectrometry after extraction from human serum and derivatization with 2,2,2-trichloroethyl chloroformate. Aniline, along with the internal standard N-methylaniline, were extracted from alkaline serum using chloroform. Aniline and the internal standard were derivatized with 50 microl 2,2,2-trichloroethyl chloroformate. After evaporating excess derivatizing reagent, the residue was reconstituted in 50 microl chloroform and injected into the gas chromatographic-mass spectrometry (GC-MS) system. A positive identification of derivatized aniline can be made by observing strong molecular ions at m/z 267 and 269. Similarly, the derivatized internal standard showed strong molecular ions at m/z 281 and 283. The within-run and between-run precisions of the assay were 3.61 and 5.92%, respectively, at an aniline concentration of 5 mg/l. The assay was linear for serum aniline concentrations of 0.5-25.0 mg/l. The detection limit was 0.1 mg/l. The assay was not affected by lipemia, hemolysis or high bilirubin concentration in serum.