The interaction of deoxyribonucleoside-5'-mono-, di- and triphosphates with E. coli DNA polymerase I Klenow fragments was examined. Dissociation constants of the enzyme complex with nucleotides were determined from the data on the enzyme inactivation by adenosine 2',3'-riboepoxide 5'-triphosphate. The role of nucleotide bases, phosphate groups and sugar moieties in the complex formation of nucleotides with the enzyme was elucidated. The necessity of complementary interaction of nucleotides with templates for template-controlled 'adjusting' of complementary dNTP to its reactive state was found. The crucial role of the interaction of dNTP gamma-phosphate with the enzyme in this process is discussed.