Usambarensine is a plant alkaloid isolated from the roots of Strychnos usambarensis collected in Central Africa. This bis-indole compound displays potent antiamoebic activities and shows antigardial, antimalarial and cytotoxic effects. Usambarensine is highly toxic to B16 melanoma cells and inhibits the growth of leukemia and carcinoma cells. To date, the molecular basis for its diverse biological effects remains totally unknown. However, its capacity to inhibit nucleic acids synthesis in melanoma cells, on the one hand, and its structural analogy with DNA-binding pyridoindole plant alkaloids recently studied (cryptolepine and matadine), on the other hand, suggested that usambarensine could also bind to DNA. Consequently, we studied the strength and mode of binding to DNA of usambarensine by means of absorption, circular and linear dichroism. The results of the optical measurements indicate that the alkaloid effectively binds to DNA and behaves as a typical intercalating agent. Biochemical experiments indicated that, in contrast to cryptolepine and matadine, usambarensine does not interfere with the catalytic activity of topoisomerase II. Human HL60 leukemia cells were used to assess the cytotoxicity of the alkaloid and its effect on the cell cycle. Usambarensine treatment is associated with a loss of cells in the G1 phase accompanied with a large increase in the sub-G1 region which is characteristic of apoptotic cells. The DNA of usambarensine-treated cells was severely fragmented and the proteolytic activity of DEVD-caspases is enhanced. Usambarensine is thus characterized as DNA intercalator inducing apoptosis in leukemia cells.