Selenium status was assessed in 28 infants receiving either human milk (n = 8) or a proprietary formula (n = 20) as their sole nutritional source from birth to 3 months. Infants' selenium intakes were determined from dietary records, "test weighings," and direct analysis of human milk and formula. Fore milk samples (n = 72) collected from group I mothers had a mean selenium concentration (ng/ml) of 16.3 +/- 4.9, whereas the selenium content of proprietary formula was 8.6 +/- 0.9. The selenium concentration of hind milk was significantly greater (p less than 0.05) than that of fore milk. At 3-months selenium intakes as well as serum selenium concentrations for human milk fed infants were significantly greater than those for formula fed infants. A significant positive correlation (r = 0.42; p less than 0.05) was found between infants' selenium intakes and their serum selenium concentrations. The results indicate that feeding practice directly affects selenium status during infancy.