The selenium content of human milk, cow's milk and cow's milk infant formula were estimated by instrumental neutron activation analysis. The highest values were found in 3 samples of human colostrum (524--865 x 10(-9) g/g dry weight). There was a significant decrease with increasing time post partum. Mature human milk exhibited a selenium content of 230 +/- 79 x 10(-9) g/g dry weight. The selenium content of 45 samples of cow's milk from the north-western area of Germany was 200 +/- 39 x 10(-9) g/g dry weight. While there was no significant difference between the values of mature human milk and of cow's milk, cow's milk infant formula exhibited significantly (P less than 0.01) lower values than human milk. The average selenium content of 107 samples of 10 different commercially available fluid and powdered cow's milk infant formulas (range: 18--171 x 10(-9) g/g dry weight) amounted to about only one third of that in mature human milk.