Captan inhibits DNA polymerases of both eukaryotic and prokaryotic sources. When polymerases were employed in assays with various nucleotides as template-primer, no specificity in the base sequence of polynucleotide was required for inhibition. Sucrose gradient centrifugation and preincubation studies showed the inhibition was caused by an irreversible alteration of the polymerase. Captan and DNA compete for the same site on the polymerase, thus DNA can serve a protective role in the elimination of captan's action. The pyrophosphate exchange activity associated with the polymerase is not inhibited by captan and the fidelity with which DNa polymerase I copies the DNA template also is not altered by captan treatment.