Glutamine 5-phosphoribosyl-1-pyrophosphate amidotransferase (EC 2.4.2.14), amidophosphoribosyltransferase, was partially purified from human placenta. Upon exposure to oxygen, both the glutamine and ammonia activities were lost in parallel. Inactivation by oxygen increased as the temperature of incubation rose and the partial pressure of oxygen increased. Molecular oxygen rather than a radical derivative was responsible for inactivation since scavengers of oxygen radicals did not protect against inactivation. AMP,GMP,PP-ribose-P, and inorganic phosphate partially protected both the glutamine and ammonia activities from inactivation by oxygen. Incubation with 1,10-orthophenanthroline, but not 1,7-metaphenanthroline or tiron, led to inactivation of amidophosphoribosyltransferase. Both the 1,10-orthophenanthroline- and oxygen-inactivated enzymes could be reconstituted by incubation with ferrous iron and inorganic sulfide in the presence of dithiothreitol under anaerobic conditions. The iron requirement could not be replaced by zinc, copper, cobalt, nickel, magnesium, or calcium. The sulfide requirement could not be replaced by higher concentrations of dithiothreitol. It is concluded from these studies that human amidophosphoribosyltransferase is an iron-sulfur protein and oxidation of this structure may be responsible for the marked lability of this enzyme in vitro.