Quinones with one, two and three aromatic rings are a new class of micromolar non-peptidic inhibitors of HIV-1 proteinase, an enzyme essential for replication of Human Immunodeficiency Virus and an important drug target for AIDS. Substituted anthraquinones bearing hydroxyl substituents on one of their three rings were the most potent of these inhibitors. Comparisons with other small non-peptidic inhibitors that are now emerging, together with enzyme kinetic data indicating that alizarin is a competitive inhibitor, suggest that anthraquinones bind in the active-site groove of HIV-1 proteinase.