In the search for 1-[(2-hydroxyethoxy)-methyl]-6-(phenylthio)thymine (HEPT) derivatives, we have found several 5-ethyl-6-(phenylthio)uracil analogues to be highly potent and selective inhibitors of human immunodeficiency virus (HIV) type 1. 1-Benzyloxymethyl-5-ethyl-6-phenylthiouracil, the most potent congener of the series, inhibits HIV-1 replication in a variety of cell systems, including peripheral blood lymphocytes, at a concentration of 1.5-7.0 nM, which is lower by a factor of 10(3) than the 50% antivirally effective concentration of the parent compound HEPT. The 5-ethyl-6-(phenylthio)uracil analogues, like HEPT itself, do not inhibit HIV-2 replication but do inhibit replication of 3'-azido-3'-deoxythymidine-resistant mutants of HIV-1. 1-Benzyloxy-methyl-5-ethyl-6-phenylthiouracil and its congeners are targeted at the HIV-1 reverse transcriptase (RT). They do not inhibit HIV-2 RT. They do not need to be metabolized to exert their inhibitory effect on HIV-1 RT. Yet this inhibitory effect is competitive with the natural substrate dTTP. The HEPT derivatives represent a group of RT inhibitors with a unique mode of interaction with HIV-1 RT.