The base analog 6-N-hydroxylaminopurine (HAP) induces bidirectional GC-->AT and AT-->GC transitions that are enhanced in DNA polymerase epsilon and delta 3'-->5' exonuclease-deficient yeast mutants, pol2-4 and pol3-01, respectively. We have constructed a set of isogenic strains to determine whether the DNA polymerases delta and epsilon contribute equally to proofreading of replication errors provoked by HAP during leading and lagging strand DNA synthesis. Site-specific GC-->AT and AT-->GC transitions in a Pol+, pol2-4 or pol3-01 genetic background were scored as reversions of ura3 missense alleles. At each site, reversion was increased in only one proofreading-deficient mutant, either pol2-4 or pol3-01, depending on the DNA strand in which HAP incorporation presumably occurred. Measurement of the HAP-induced reversion frequency of the ura3 alleles placed into chromosome III near to the defined active replication origin ARS306 in two orientations indicated that DNA polymerases epsilon and delta correct HAP-induced DNA replication errors on opposite DNA strands.