The clinical antileukemic drug amsacrine and analogues are thought to exert their biological activity by binding tightly but reversibly to DNA, with the acridine chromophore intercalated and the anilino group making additional binding contact in the minor groove of the double helix. In this binding model the steric environment around the 3'- and 5'-positions of the anilino ring is crucial. Two 3',5'-disubstituted analogues of amsacrine have been prepared, and their conformation, DNA binding properties, and antitumor activity were determined and compared with corresponding unsubstituted and 3'-substituted compounds. Addition of 3'- and 3',5'-substituents have little effect on minimum-energy conformations of the anilino side chain but have significant effects on DNA binding and biological activity. Monosubstitution lowers binding constants several-fold, but intercalative binding with extensive drug-base pair overlap is retained. Disubstitution lowers binding further, and although the binding is still intercalative as assessed by unwinding angles, it appears to occur with little drug-base pair overlap, as determined by high-field NMR studies of DNA imino proton shifts. These changes in DNA binding are accompanied by an abrupt change in biological activity, with the 3',5'-disubstituted analogues proving inactive and nontoxic even though other physicochemical properties, such as lipophilicity and stability, remain within acceptable limits. This study provides further evidence that the binding of drugs to DNA has a critical influence on their biological activity.