A procedure for the simultaneous quantitative carbamidomethylation of selenocysteine (SeCys) and selenomethionine (SeMet) followed by their proteolytic release from blood proteins was developed. The fraction containing both derivatized selenoaminoacids was isolated by size-exclusion chromatography and submitted to ion-pairing HPLC-ICP MS analysis. The limit of detection was ca. 0.02 ? g g-1 (dry mass) for either amino acid. The quantification of SeCys and SeMet was carried out by the method of standard additions. An internal standard of 77Se-labelled SeMet was used to control the derivatization yield and chromatographic recovery. The determination of SeCys was validated by spiking with glutathione peroxidase. An additional proof of validity was achieved by monitoring the selenium mass balance (12 series of analysis over a period of 18 months; the Se amino acids accounted for 92 ± 8% of the total Se). The method was applied to the monitoring of changes in SeCys and SeMet concentrations in lamb blood during supplementation studies (tolerance and dose effect) with selenium-rich yeast. © The Royal Society of Chemistry 2008.