Halenaquinol sulfate, a p-hydroquinone sulfate obtained from a marine sponge, inhibited the activity of eukaryotic DNA polymerases in varying degrees; the K, values for DNA polymerases a, /3, 6 and E were 1.3, 80, 17.5 and 2.0 ,uM, respectively, whereas it was less effective against E. co/i DNA polymerase I. The inhibition occurred competitively with each of dATP and dTTP, but non-competitively with dCTP, dGTP and the template DNA. Thus, halenaquinol sulfate is demonstrated to be a potential inhibitor of DNA polymerases a and E, and be a useful tool for analyzing the dNTP binding sites of DNA polymerases.