Globin samples from ethylene oxide-exposed workers and non-exposed referrents were analysed by two methods: (i) gas chromatography-mass spectrometry determination of Nt-(2-hydroxyethyl)histidine as its methyl ester heptafluorobutyryl derivative, after hydrolysis of the protein and isolation of the alkylated amino acid by ion exchange chromatography. The internal standard, Nt-(2-hydroxy-d4-ethyl)histidine, was added to the protein before hydrolysis. (ii) Determination of N-(2-hydroxyethyl)valine after derivatization of the protein by a modified Edman procedure, extraction and g.c.-m.s. determination of alkylated N-terminal valine in the form of its pentafluorophenylthiohydantoin derivative. The internal standard used was in this case a globin with a known content of hydroxy-d4-ethylated amino acids. The two methods gave consistent results, especially at high levels of alkylated products. The average content of hydroxyethylhistidine was 0.6 nmol/g higher than the content of hydroxyethylvaline. Higher levels of background alkylation (of unknown origin) were recorded with the histidine method as compared with the valine method, suggesting that the latter assay should show greater sensitivity for low level ethylene oxide exposure monitoring.