The concentration of selenium (Se) in milk samples obtained from 26 lacto-ovo-vegetarian (vegetarian) women was significantly greater (22.2 +/- 0.8 ng/ml) than from 12 nonvegetarian women (16.8 +/- 1.3). Mean GSH-Px activity (EC 1.11.1.9) in milk from vegetarians was 146% of that in milk from nonvegetarians. A significant correlation between GSH-Px activity and Se concentration was observed (r = 0.76). Likewise a significant correlation was observed between milk linoleic acid content and GSH-Px activity (r = 0.68). In undialyzed samples from vegetarian women, most of the increased content of Se was found in fractions containing proteins of 100 kdaltons (kD) or more. The high glutathione peroxidase activity in milk from vegetarians was associated with selenoproteins in the 90 to 100 kD range. A distinct 40 kD Se peak was also detected in milk from vegetarians. There was no difference in Se intake between the two groups of women. Therefore, although data from the present study show that both the quantity and distribution of Se in human milk can be modified by the maternal diet, the increased Se content and GSH-Px activity in milk from vegetarians cannot be explained by an increased Se intake.