This paper describes the powerful cytotoxic action exerted by strychnopentamine (SP), a dimeric indole alkaloid extracted from Strychnos usambarensis Gilg, on B16 melanoma cells and on non-cancer human fibroblasts cultured in vitro. SP strongly inhibits cell proliferation and induces cell death at a relatively low concentration (less than 1 microgram/ml) after 72 h of treatment in the two lines. Incorporation of [3H]thymidine and [3H]leucine by B16 cells significantly decreases after only 1 h of treatment at 0.5 microgram/ml. SP induces the formation of dense lamellar bodies and vacuolization in the cytoplasm, intense blebbing at the cell surface and various cytological alterations leading to cell death.