The hydroxamic acid bond occurs in products from fungi, yeast, bacteria, and plants. The -CON(OH)-bond arises by oxidation of a free or bound amino group in a unit structure which is often closely related to conventional amino acids. Products are known with one, two, or three hydroxamic acid groups per molecule. The chemistry of the ferrichrome type compounds, which are ferric trihydroxamate-containing peptides, has been worked out in detail and includes a complete crystallographic analysis of the ferrichrome A molecule. The trihydroxamates form potent complexes with ferric ion, called siderochromes, and these are believed to play a role in the metabolism of the metal ion in microorganisms. The actual physiological activity observed ranges from that of growth factor, antibiotic, antibiotic antagonist, tumor inhibitor or cell-division factor. The precise molecular mechanism whereby these substances exert their potent beological activity remains to be elucidated.