When rat liver homogenate or its postmitochondrial supernatant was incubated with L-cysteine, but not D-cysteine, ornithine decarboxylase (ODC) lost more than half of its catalytic activity within 30 min and, at a slower rate, its immunoreactivity. The inactivation correlated with production of H2S during the incubation. These changes did not occur in liver homogenates from vitamin B6-deficient rats. A heat-stable inactivating factor was found in both dialysed cytosol and washed microsomes obtained from the postmitochondrial supernatant incubated with cysteine. The microsomal inactivating factor was solubilized into Tris/HCl buffer, pH 7.4, containing dithiothreitol. Its absorption spectrum in the visible region resembled that of Fe2+ X dithiothreitol in Tris/HCl buffer. On the other hand FeSO4 inactivated partially purified ODC in a similar manner to the present inactivating factor. During the incubation of postmitochondrial supernatant with cysteine, there was a marked increase in the contents of Fe2+ loosely bound to cytosolic and microsomal macromolecules. Furthermore, the content of such reactive iron in the inactivating factor preparations was enough to account for their inactivating activity. These data suggested that H2S produced from cysteine by some vitamin B6-dependent enzyme(s) converted cytosolic and microsomal iron into a reactive loosely bound form that inactivated ODC.