We have examined Ingenol derivatives, the extracts from kansui (dried roots ofEuphorbia Kansui Liou) for their inhibitory effects on HIV replication in vitro and found that they are highly potent inhibitors of the virus. In the anti-HIV assays, MT-4 cells were infected with HIV and incubated in the presence of varying concentrations of the test compounds. After a 4- or 5-day incubation period, the anti-HIV activities were determined by the inhibition of virus-induced cytopathic effect in infected cells or the reduction of p24 antigen in culture supernatants. The 50% effective concentration (EC 5 0 ) and the 50% cytotoxic concentration (CC 5 0 ) were calculated by the standard method. We have also examined the inhibitory effects of the Ingenol derivatives on syncytium (multinucleated giant cell) formation induced by cocultivation of Molt-4 cells with Molt-4/IIIB (a cell line persistently infected HIV-1) cells. The most potent compound of this series inhibited the replication of HIV-1 I I I B at a concentration of 0.1nM with a selectivity index greater than 100,000. The Ingenol derivatives were also inhibitory to the replication of several strains of HIV-1, including clinical isolates, AZT-resistant mutants, and nonnucleoside reverse transcriptase (NNRTI)-resistant mutants, and HIV-2. Syncytium formation was suppressed by the compounds. Studies on their mechanism of action suggest that the Ingenol derivatives act on the stage of viral entry into the host cells.