The study of new intercalating DNA agents has led to the synthesis of dipyrido [4,3-b] [3,4-f] indole derivatives. These compounds bind preferentially to helical DNA and their affinity constants range from 10(6) to 10(7) M-1. The most active derivative against L1210 mouse leukemia produces viscosity changes in sheared fragments of linear DNA and in circular PM2 DNA on binding, indicating an intercalation process. The unwinding angle is estimated to be 18(0). This active derivative binds preferentially to A-T base pairs.