Isostrychnopentamine (ISP) is an asymmetric indolomonoterpenic alkaloid isolated from the leaves of Strychnos usambarensis. The in vitro antiplasmodial activities against five Plasmodium falciparum cell lines were evaluated: ISP possessed an in vitro IC (50) near 0.1 microM against all P. falciparum cell lines tested (chloroquine-resistant and chloroquine-sensitive lines) and showed antiplasmodial selectivity compared to cytotoxicity on human cell lines. The stage-dependent susceptibility to a short exposure of ISP was then investigated. The ring stage was shown to be the most sensitive one, but all stages were affected by ISP treatment. By means of fluorescence microscopy, it was shown that ISP was not accumulated inside the food vacuole of the parasite. Finally, the in vivo antimalarial activities against the P. berghei NK173 and P. vinckei petteri murine strains were determined. The ED (50) in vivo was about 30 mg/kg/day by the intraperitoneal route (after 4 days treatment).