Enzymic activity and protein levels of xanthine oxidase were measured in serial samples of breast milk donated by each of 14 mothers, starting, in all but two cases, within 7 days following parturition. Enzyme activity varied widely, usually reaching peak values during the first 15 days and falling thereafter, by as much as 98%, to basal levels that were subsequently largely maintained. Corresponding changes in xanthine oxidase protein levels were not observed and, consequently, the specific activity of xanthine oxidase followed the above pattern. The capacity of human xanthine oxidase to undergo activation-deactivation cycles at the molecular level has important implications, not only for its role in breast milk, but also for its potential as a source of reactive oxygen species in other human tissues.