An increase in the binding affinity of rat liver trans-acting nucleoprotein p70 for the hormone responsive element of the rat haptoglobin gene in acute-phase reactions has implicated a posttranslational modification. This investigation examines the proposed acute-phase related structural alterations of p70 using an in vitro phosphorylation/dephosphorylation assay and selective digestion of p70 with Staphylococcal aureus V8 protease. The results show that p70 requires phosphorylation to express its DNA-binding ability. Selective proteolysis of p70 provided evidence that acute-phase induced phosphorylation of this protein alters its conformation in such a way that its DNA-binding ability is increased.