A simple 1+15 dilution of 200 µl volumes of serum with a diluent containing 1.0% v/v butan-l-ol; 0.66% Triton X-100; 0.01 mol l ^-1 NH ₃ , 0.0002 mol l ^-1 (NH ₄ ) ₂ H ₂ EDTA and 0.002 mol l ^-1 NH ₄ H ₂ PO ₄ enables ICP-MS to provide accurate and precise measurements of total selenium concentrations. Interferences from argon-adduct ions on selenium isotopes ⁷⁷ Se, ⁷⁸ Se are eliminated and are considerably reduced at ⁸² Se by the presence in the diluent of butan-l-ol. The other reagents prevent blockage of the torch injector tube during prolonged, ≥6 h, analytical runs. The detection limit (3 s blank) is 0.02 µmol l ^-1 (1.5 µg l ^-1 ) and within-run RSDs range from 3.4 to 7.5% at 0.3 µmol l ^-1 , to 1.5 to 2.0% at 2.1 µmol l ^-1 . Analyses of 12 sera from an external quality assessment program, of a commercially available reference serum material (Nycomed) and of in-house internal quality control sera gave excellent agreements with median/target concentrations over the range 0.3 to 3.3 µmol l ^-1 . The ICP-MS data showed small, but acceptable, negative biasses at 1.0 µmol l ^-1 of: -3.8% and -5.0% at ⁷⁷ Se and ⁷⁸ Se, respectively.