We developed a two-site immunoradiometric assay for rat serum ferritin that uses antibody immobilized on agarose. Individual serum ferritin values were significantly correlated with iron stores as determined chemically by liver ferritin iron content. This group correlation was not sufficiently great, however, to allow confident prediction of iron stores in a given animal on the basis of serum ferritin alone. Significant differences in mean liver ferritin iron concentration between groups of rats raised on diets of differing iron content were not always reflected by differences in mean serum ferritin values. Data correlating the serum levels of ferritin and a liver-specific transaminase suggested that hepatocellular death may sometimes contribute ferritin to the serum. Strong postitive correlations between serum ferritin and iron stores in the rat were not observed when serum transaminase levels were in the normal range.