Plasma selenium concentration and glutathione peroxidase activity in red blood cells were determined in subjects from different age groups. The selenium level (mean ± SD) found in infancy (0 to 6 months) was 2 ± 0.6 /μg/dl, with the lowest value of 1 /μg/dl observed in a 4- month-old infant. These levels were significantly lower (p<0.001) than the value of 9.5 ± 1.1 /μg/dl found in the adult group and 7.7 ± 1.3 μg/dl found in the group of older children (2 to 15 yr). Younger children (6 to 24 months of age) had intermediate levels of 5 ± 1.2 /μg/dl. When the data were plotted on a logarithmic scale as a function of age, the figure shows clearly that the plasma selenium levels increase steadily with age throughout life after an initial drop at 60 to 90 days. There was a satisfactory correlation between the plasma selenium concentration and the enzyme glutathione peroxidase activity in the red blood cells (Spearman's ρ=0.45, p<0.005). Although very low selenium values were observed, the enzyme glutathione peroxidase activity remained above 10 U/g hemoglobin (with only one exception) in all patients.