Glutamine phosphoribosylpyrophosphate amidotransferase, purifed to better than 98% purity from derepressed Bacillus subtilis, exists as a tetramer and as a dimer of apparently identical subunits with a molecular weight of 50,000 each. The enzyme contains 3 atoms of iron and 2 atoms of inorganic sulfide per subunit and has a yellow-brown color. The absorption spectrum is not altered by dithionite, but exposure to oxygen causes inactivation and partial bleaching of the visible spectrum. Thus, the Bacillus amidotransferase exhibits novel structural features and a new reaction type of proteins of the iron-sulfur group.