The selenium content in blood was determined using the hydrogen catalytic peak. This peak at -1.1 V was obtained in the presence of selenium and molybdenum at pH values of 1-4 in different buffers. For the determination of selenium, the Mo(VI) concentration has to be similar to 100-200 times higher than the selenium present. The linear domain range of selenium is 1 x 10(-6)-5 x 10(-9) M. The interference of zinc is eliminated by the addition of EDTA at pH 3.5 acetate buffer. The method was applied to 1.0 mi of digested blood, and 620 +/- 44 mu g l(-1) Se and 7.15 mg l(-1) Zn could be determined with a 90% (n = 6) confidence interval. (C) 1999 Elsevier Science B.V. All rights reserved.