The 2-amino substituted derivatives of guanine, N2-(p-n-butylphenyl)guanine (BuPG) and N2-(3',4'-trimethylenephenyl) guanine (TMPG), were synthesized and found to selectively inhibit, respectively, HeLa cell DNA polymerase alpha (po1 alpha) and B. subtilis DNA polymerase III (po1 III). Both purines, like their corresponding uracil analogs, BuAu and TMAU (2,9), were specifically competitive with dGTP in their inhibitory action on their target polymerases. BuPG, the pol alpha-specific purine, was also toxic for HeLa cells in vivo, selectively inhibiting DNA synthesis. These N2-substituted purines, in contrast to the 6-substituted uracils, provide a structural basis for the synthesis of nucleosides and nucleotides with considerable potential as probes for the analysis of the structure of specific replicative DNA polymerases and their function in cellular DNA metabolism.