It is suggested that the biological activity of a variety of cationic agents against experimental leukemias and trypanosomal species is dependent on (a) lipophilic-hydrophilic balance of the agents, (b) charge separation, (c) sufficient “contact” binding, this requirement varying with the biological test system involved, (d) a close over-all approach to planarity, and (e) capacity to fit a curved site of approximately 40 Å in diameter. That migration of these materials may be transport mediated is discussed. It is shown that a site equivalent to the minor groove in a helical polynucleotide would match the structural requirements. © 1969, American Chemical Society. All rights reserved.