An inhibitory effect of 3'-azido-3'-deoxythymidine triphosphate on hepatitis B virus DNA polymerase was found. No effect was seen by its threo analog. The effect was detected at and above a concentration of 0.05 microM. The inhibition of DNA polymerase activity was the same in ten different strains of HBV. The mechanism was shown to be competitive, with an inhibition constant (Ki) of approximately 0.04 microM, and the Km value for dTTP was 0.1 microM.