A method has been developed which separates the three major selenium-containing proteins found in human blood serum and plasma: selenoprotein-P, glutathione peroxidase and albumin. They were separated from plasma or serum by affinity chromatography and the Se content determined directly by ETAAS. Selenoprotein-P is retained on a heparin-Sepharose column, and subsequently eluted with an excess of heparin, while glutathione peroxidase is separated by a blue-Sepharose column. The amount of Se associated with albumin was assumed to be the Se remaining in the rest of the sample. The detection limit of the ETAAS method, when applied to the separated fractions, was 0.8 µg l–1(2 ng absolute) and the accuracy of the determination was confirmed by comparison with spectrofluorimetry. The distribution of Se in the serum or plasma of 21 healthy people was determined, showing that 53 ± 6% of the total present is associated with selenoprotein-P, 39 ± 6% as glutathione peroxidase and 9 ± 4% as albumin.