Three highly purified DNA polymerases, Escherichia coli polymerase I (enzyme A) and the polymerases induced by wild-type T4 phage and by T4 phage mutant L141 (antimutator phenotype), have been examined with respect to their tendency to incorporate the deoxyribonucleotide of 2-aminopurine [(AP)] for deoxyadenylate at specific sites in deoxyribonucleic acid (DNA). Using phi X174 phage DNA as a template and selected phi X174 restriction fragments as specific primers, we synthesized short sequences of phi X174 DNA in vitro by the polymerase of interest, with the 5'-triphosphate of 2-aminopurine deoxyriboside and dATP at equimolar concentration. The relative incorporation of (AP) at the various adenine sites was determined by providing the newly synthesized DNA fragment with a specific terminal radioactive label, subjecting the DNA fragment to thermal depurination as a DNA cleavage reaction highly selective for (AP), and analyzing the resulting radioactive fragments by denaturing gel electrophoresis, autoradiography, and microdensitometry. The L141 polymerase shows very pronounced site-dependent variations in (AP) incorporation. For the wild-type T4 polymerase, the pattern of (AP) incorporation follows the biases seen for the L141 enzyme, although in a less pronounced form. Sequence preferences for (AP) incorporation are least marked for E. coli polymerase I (enzyme A); in several instances, they run counter to the sequence biases observed with the T4 enzymes. For the enzyme showing the most pronounced sequence effects, L141 polymerase, the extent of (AP) incorporation was determined at 57 different sites. No simple principle governing the sequence dependence of (AP) incorporation could be deduced from these results.