A study of the inhibition of mouse cellular DNA polymerases by poly-nucleotides and their vinyl analogs is presented. Poly(dT)-directed poly(dA) synthesis by representatives of all three classes of cellular DNA polymerase could be completely inhibited by poly(9-vinyladenine), although higher concentrations were required in the case of the gamma class enzyme. Studies on the mechanism of the inhibition using the alpha class DNA polymerase and different templates showed that the enzyme activity was inhibited in all cases where base-pairing between the vinyl polymer and the template occurred; poly(9-vinyladenine) did not interfere with the replication of templates to which it does not bind. The inhibition occurred shortly after addition of poly(9-vinyladenine) to ongoing reactions, yet the enzyme was not displaced from the template - primer complex.