The contribution of counterions to macroscopic properties of isotropic DNA solutions has been studied using osmotic pressure measurements in low added salt condition. In the high DNA concentration range, the counterion contribution prevails and the associated osmotic coefficient is equal to $0.245\pm 0.020$. In the lower concentration range, the osmotic pressure may be exerted either by polymers or by ions, or due to a combination of both effects, depending on the added salt and DNA concentrations.

• Received 21 June 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.2533