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Human milk contains various bioactive compounds including numerous immunologic factors, enzymes, growth factors, and hormones. However, the change during the course of lactation in many of these compounds has not been fully characterized. Therefore, the objective of the present study was to measure the activity of the enzymes superoxide dismutase (SOD; Enzyme Commission number [EC] 22.214.171.124) and glutathione peroxidase (SeGSHPx; EC 126.96.36.199) in human milk, to record changes in enzyme activity over time and to determine whether there are differences in activity between the milk of mothers of full-term (FT) and premature (PT) infants.Nine samples were collected from each of 15 mothers (32 ± 4 years of age; mean ± standard deviation) of FT infants (gestational age, 40 ± 1 weeks; birth weight, 3544 ± 417 g) and 19 mothers (28 ± 5 years of age) of healthy PT infants (gestational age, 29 ± 4 weeks; birth weight, 1312 ± 479 g). Samples were collected within a week of birth (±1 day) and thereafter for 8 weeks, with a final collection at 12 weeks.During the 12-week study period, in both groups, total milliunits of GHSPx and SeGHSPx per milligram protein and SOD per per milligram protein increased, whereas protein content declined. SeGHSPx per milliliter milk was higher in the PT group at week 1 (92 ± 30 mU/mL vs. 73 ± 21 mU/mL), week 2 (93 ± 28 mU/mL vs. 75 ± 24 mU/mL), and week 7 (85 ± 24 mU/mL vs. 68 ± 22 mU/mL). The SOD activity per milliliter milk and milligram protein was higher throughout the entire study in the FT milk.Because mothers of PT infants may produce less milk than those of FT infants, PT infants may be at a disadvantage for antioxidant protection from these enzymes.