The inhibitory effect of suramin on the phosphorylation of GST-HBV core fusion protein (GST-Hcore) and two GST-Hcore fusion polypeptides (Hcore157B and Hcore164B) by two α-type cAMP-dependent protein kinases (PKAIα and PKAIIα) was biochemically investigated in vitro. It was found that (i) this phosphorylation was inhibited by suramin at a low concentration (IC₅₀=approx. 10 nM); (ii) a relative high dose of suramin was required to inhibit an autophosphorylation of PKAIIα (IC₅₀=approx. 0.7 μM) and the PKAIIα-mediated phosphorylation of histone H2B (IC₅₀=approx. 0.4 μM); (iii) the PKAIIα-mediated phosphorylation of Hcore157B was more sensitive to suramin than the phosphorylation of Hcore157B by Ca²⁺-dependent protein kinase (PKC); and (iv) suramin had a high binding affinity for Hcore157B, but not for histone H2B in vitro. These results suggest that suramin selectively inhibits the PKA-mediated phosphorylation of HBV-CP through the direct binding in vitro of suramin to the Arg-rich C-terminal region (containing three potential phosphorylation sites for PKA) on HBV-CP.