Iron-responsive element-binding protein (IRE-BP) activity was studied in liver and intestinal samples of hemochromatosis and control patients using a short 32P-IRE-RNA probe on “retardation” nondenaturing polyacrylamide gels. IRE-BP activity was assessed in liver biopsy specimens in 36 patients—16 hemochromatosis homozygotes, 4 hemochromatosis heterozygotes, 6 patients with secondary iron overload, and 10 control patients with normal hepatic iron concentrations. Intestinal IRE-BP activity was assessed in 14 hemochromatosis homozygotes and 16 normal subjects. Endogenous IRE-BP activity was determined from 32P retarded on the gel, and total IRE-BP activity was assessed after reducing tissue samples with 2-mercaptoethanol. Hepatic endogenous IRE-BP activity was inversely related to hepatic iron concentration (r = −.59, P < .0002). Mean hepatic endogenous IRE-BP activity in the hemochromatosis homozygotes, 0.25 ± 0.04 pmol/mg protein, was also significantly decreased compared with values in the normal controls, 0.45 ± 0.06 pmol/mg protein, P < .05. Hepatic total IRE-BP was also significantly decreased in the hemochromatosis patients by gel retardation assay and Western blotting with anti-IRE-BP antibody. Intestinal endogenous IRE-BP activity, total IRE-BP activity, and iron concentration did not significantly differ between hemochromatosis patients and normal control subjects. This suggests that both endogenous IRE-BP activity and the total amount of the protein are downregulated in the liver by tissue iron. Intestinal IRE-BP activity that regulates intestinal transferrin receptor expression is normal in hemochromatosis and appropriate for the intracellular iron concentration. (Hepatology 1995; 22:828–832.)