In the iron-loaded liver there may be an increase in the putative intracellular transit pool of iron, components of which could be catalytically active in stimulating lipid peroxidation. To study the levels of low-molecular-weight, catalytically active iron in the liver, cytosolic ultrafiltrates were tested in an assay containing rat liver microsomes and NADPH. Malondialdehyde production was used as an index of lipid peroxidation. This assay system was sensitive enough to detect 0.25 pmol/L ferrous iron; progressive but nonlinear increases in malondialdehyde were produced as the iron concentration was increased to 5 FmoUL. Ultrafiltrates from hepatic cytosol of iron-loaded rats had greater prooxidant action than did those from controls. When added to the as~ay, deferosamine, an iron chelator, completely suppressed the prooxidant action of hepatic ultrafiltrates, showing that this activity is iron-dependent. Deferoxamine administered intraperitoneally to control animals at a dose of 1 gm/kg completely inhibited the prooxidant effect of hepatic ultrafiltrates prepared from rats killed after 1, 2 and 3 hr. Partial inhibition was observed at 4 hr; by 6 hr the inhibitory effect of deferoxamine was completely lost. Administration of deferosamine ( 1 gmkg intraperitoneally, 1 hr before killing) completely inhibited the prooxidant action of hepatic ultrafiltrates in moderately iron-loaded rats and controls but had no protective effect in heavily iron-loaded rats. These results support the concept that iron overload results in an increase in a hepatic cytosolic pool of low-molecularweight iron that is catalytically active in stimulating lipid peroxidation. This pool can be chelated transiently in v i m by deferosamine in moderate, but not heavy, iron overload.