New drugs and molecular targets are needed against Trypanosoma brucei, the protozoan that causes African sleeping sickness. Tryptanthrin (indolo[2,1-b]quinazoline-6,12-dione), a traditional antifungal agent, and 11 analogs were tested against T. brucei in vitro. The greatest activity was conferred by electron-withdrawing groups in the 8 position of the tryptanthrin ring system; the most potent compound had a 50% effective concentration of 0.40 microM.