Deoxyguanosine-5'-monophosphate in water self-associates into stable structures, which include liquid-crystalline hexagonal and cholesteric phases. The structural unit is a four-stranded helix, composed of stacked Hoogsteen-bonded guanosine quartets. By using the osmotic stress method, we recently measured the force between helices in KCl solutions up to 2 M. In addition to the long-range electrostatic force, a short-range hydration repulsive contribution was recognized. The hydration repulsion is exponential, and shows a decay length independent from the ionic strength of the solution. Here, we report that more concentrated KCl solutions cause condensation of the guanosine helix in a hexagonal phase with constant equilibrium separation of approximately 7 A between helix surfaces. Long-range attraction, which induces the self-assembly, and short-range repulsion, which prevents the contact between the helices, are implied. By using osmotic stress, the force needed to push helices closer from the spontaneously assumed position has been measured. The attractive force was then estimated as a difference between the net force and the repulsive contribution, revealing an exponential decay length about two times larger than that of the short-range repulsion. The agreement with the helix interaction theory introduced recently by Kornyshev and Leikin (Kornyshev, A. A., and S. Leikin, 1997. Theory of interaction between helical molecules. J. Phys. Chem. 107:3656-3674) suggests that the repulsive and attractive forces originate from helix-specific interactions.