The G-quadruplex structural motif of DNA has emerged as a novel and exciting target for anticancer drug discovery. The human telomeric G-quadruplex consists of a single strand repeat of d[AGGG (TTAGGG)3] that can fold into higher-order DNA structures. Small molecules that selectively target and stabilize the G-quadruplex structure(s) may serve as potential therapeutic agents and have garnered significant interest in recent years. In the work presented here, the anticancer agent, actinomycin D, is demonstrated to bind to and induce changes in both structure and stability in both the Na+ and K+ forms of the G-quadruplex DNA. The binding of actinomycin D to the G-quadruplex DNAs is characterized by intrinsic association constants of approximately 2 × 105 M-1 (strand) and 2:1 molecularity, and are shown to be enthalpically driven with binding enthalpies of approximately -7 kcal/mol. The free Na+ or K+ forms of the quadruplex structures differ in melting temperatures by approximately 8 °C (60 and 68 °C, respectively), whereas both forms, when complexed with actinomycin D are stabilized with melting temperatures of approximately 79 °C. The induced CD signals observed for the actinomycin D-G-quadruplex complexes may indicate that the phenoxazone ring of actinomycin D is stacked on the G-tetrad rather than intercalated between adjacent G-tetrads. Complex formation with actinomycin D results in changes to both the Na+ or K+ structural isoforms to ligand-bound complexes having similar structural properties and stabilities. © 2009 American Chemical Society.