A simple method for measuring selenium in serum by ICP-MS using a 1+14 dilution of a 100 µl sample in a diluent containing 1.0% v/v butan-1-ol to eliminate interferences at ⁷⁸ Se from argon adduct ions has been shown to be accurate and precise in a 24 month evaluation for routine clinical assays and has been developed to allow analysis of whole blood and erythrocytes. Excellent performances were achieved in two External Quality Control Assessment programmes for the measurement of selenium in serum: Centre du Toxicologie de Quebec and Trace Element Quality Assessment Scheme from the University of Surrey (Guildford). The regression equation for our ICP-MS data versus target values for analysis of Se in 109 sera from November 1996 to March 1998 was: ICP-MS=0.969x Target+0.031 µmol l ^–l , r=0.996. For serum analysis the detection limit was 0.02 µmol l ^–1 , and the RSDs at 0.3-2.0 µmol l ^–1 were 2.0-4.0% within-run and 4.3-10.0% between-run. Initial applications of this method to measuring selenium in whole blood (1+14 dilution) and red blood cells (1+29 dilution) gave detection limits of 0.02 µmol l ^–1 ; RSDs of 2.1-5.8% within-run, and of 5.0-8.1% between-run, at concentrations of 0.3-2.0 µmol l ^–1 . Analysis of a whole blood reference sample (Seronorm 404107), with a target concentration of 1.01 µmol l ^–1 and acceptable range of 1.01-1.15 µmol l ^–1 , gave 1.06±0.06 µmol l ^–1 (mean±s).