Studies on the isolation and characterization of Proteus mirabilis siderophores provided no evidence that these bacteria synthesize catechol- or hydroxamate-type siderophores. However, gas chromatograph analysis in conjunction with mass spectroscopy revealed the presence of alpha-hydroxyisovaleric acid, a previously unknown metabolite. Additional substantiating evidence for the presence of alpha-hydroxyisovaleric acid in these bacteria was obtained from experiments involving the use of thin-layer chromatography and an ultraviolet absorption spectrum. This compound was found to be capable of removing iron from the synthetic chelator, ethylene-diamine-di-orthohydroxyphenyl acetic acid, and supplying that iron to the bacteria both in a solid agar medium and in a liquid medium. Proteus mirabilis was found to possess an enzyme capable of catalyzing the reaction by which alpha-hydroxyisovaleric acid is converted to alpha-ketoisovaleric acid, an intermediate in the valine biosynthetic pathway.