In order to examine the levels of serum selenium in Europe, a collaborative study was conducted under the auspices of "The Working Group on Diet and Cancer" under "The European Organisation for Cooperation in Cancer Prevention Studies". A total of 502 serum samples was obtained from healthy, non-institutionized individuals, aged between 20 and 65 years, from 17 locations in 10 different countries in Europe. The selenium content of the samples was determined by a fluorometric method. All analyses were performed in one laboratory. Mean +/- standard deviation of the serum selenium given in microgram/l for the combined male and female data from the individual regions was: Belgium: 100 +/- 9; Denmark: Aarhus 78 +/- 15; France: Grenoble 79 +/- 15; Paris 82 +/- 11; W. Germany: Bavaria 70 +/- 10 Giessen 68 +/- 10, Heidelberg 76 +/- 9; Greece 63 +/- 14; Netherlands: 93 +/- 12; Portugal: Lissabon 102 +/- 10; Spain: Barcelona 87 +/- 14; Sweden: Göteborg 77 +/- 11, Malmö 90 +/- 14, Umeå 82 +/- 8, Uppsala 81 +/- 15; United Kingdom: Ipswich 107 +/- 13, London 109 +/- 14. None of the values represented toxic or overt deficiency levels.