The epidemic of acquired immune deficiency syndrome (AIDS) continues to present an urgent requirement for new drug development candidates with antiviral activity toward the human immunodeficiency virus (HIV). The US National Cancer Institute has undertaken a major new initiative to discover novel anti-HIV agents from natural sources. In the present study, the NCI primary in vitro screen initially disclosed anti-HIV-cytopathic activity in the organic extracts of the aerial parts of the tropical liana Ancistrocladus abbreviatus (Ancistrocladaceae), collected in Cameroon in March 1987. Bioassay-guided fractionation of those extracts provided the novel atropisomeric pair of anti-HIV-cytopathic alkaloids, michellamines A (1) and B (2). Their structures were established by plasma desorption mass spectrometry, high-resolution fast-atom-bombardment mass spectrometry, and NMR (1H-1H coupling constant analyses, difference NOE experiments). Antiviral assays showed that michellamines A and B inhibit the cytopathic effects of HIV-1 on CEM-SS human lymphoblastoid target cells and HIV-2 on MT-2 target cells in vitro. Michellamine B exhibited the same potency against both HIV-1 and HIV-2, which is significant as few known anti-HIV-1 agents are active against HIV-2. The compounds also inhibit viral replication, as indicated by reduced reverse transcriptase activity, p24 antigen production, and syncytium-forming units. Michellamines are novel dimeric alkaloids with unique C-5/C-8' linkages between naphthalene and tetrahydroisoquinoline systems, more polar than known monomeric alkaloids of the family, and represent a new active chemotype for investigation in the quest for effective anti-HIV drug candidates.