The protease of human immunodeficiency virus (HIV) is an important target for antiretroviral drug therapy. The synthesis and in vitro antiviral activity of a novel protease inhibitor, DG-35-VIII, which contains an hydroxyethylhydrazide core unit, is described. DG-35-VIII had potent activity against HIV-1 and related viruses (HIV-2 and simian immunodeficiency virus) in an acutely infected T lymphocyte line (MT-2) and was also active in cells chronically infected with HIV-1, where it inhibited processing of the Pr55gag and Pr160 gag-pal precursor proteins.