Novel 3,4-dihydro-6-benzyl-4-oxopyrimidines (DABOs), variously substituted at both the C-2 and C-5 positions of the pyrimidine ring, proved to be specific inhibitors of the human immunodeficiency virus type 1 (HIV-1) in vitro. Some compounds showed potency at micromolar doses, no cytotoxicity at the maximum testable doses and selectivity indexes comparable to that of 2'-3'-dideoxyinosine (ddI). Mode of action studies suggested that DABOs interfered with a step of the virus multiplication cycle following adsorption and preceding integration. Enzyme assays indicated that DABOs targeted HIV-1 reverse transcriptase: they inhibited the RNA-dependent DNA polymerase activity in a template-dependent manner and, to a lesser extent, the DNA-dependent DNA polymerase activity. No inhibition of the RNase-H associated activity was observed. When DABOs were assayed in combination with 3'-azido-3'-dideoxythymidine (AZT) or ddI against HIV-1 in cell cultures, a slightly synergistic inhibitory effect was observed. The combination of DABO 546 and AZTTP in enzyme assays showed that the two compounds were kinetically mutually exclusive.