Derivatives of N-[2-(dimethylamino)ethyl]-9-aminoacridine-4-carboxamide bearing a wide variety of different groups at the 5-position (and for comparative purposes at the 7-position) have been prepared, and their physicochemical properties and biological activities have been determined. Although both 5- and 7-substituted compounds bind equally well to DNA by intercalation, only the 5-substituted compounds have in vivo antitumor activity. All the 5-substituted compounds showed in vivo antileukemic activity, but only those bearing electron-withdrawing substituents sufficiently powerful to ensure the acridine chromophore was uncharged at physiological pH showed activity in vivo against the Lewis lung solid tumor. The weakly basic derivatives do not show greater intrinsic cytotoxicity or selectivity toward solid tumor cells, and their broader spectrum of in vivo antitumor activity is attributed to the fact that they exist predominantly as monocations, which can distribute more efficiently.