Inhibition mechanisms of 5'-triphosphates of 3'-azido-3'-deoxythymidine (AZT-TP) and 3'-deoxythymidine (ddTTP) on extensively purified DNA polymerase gamma from bovine testes were examined by analysis of the products synthesized on singly primed M13mp18 single-stranded DNA or synthetic oligonucleotide template-primer in the presence of analogues. The results indicate that AZT-TP inhibits DNA polymerase gamma in competition with dTTP but is not incorporated into DNA, whereas ddTTP is incorporated into DNA and causes chain termination.