We have studied the conformation of poly (dG-dC) . poly (dG-dC) in three conditions; i) associated with histones octamers, ii) alone at ionic strength 0.1, and ii) in solutions of over 2.5 M NaCl. The circular dichroism spectrum for the polymer bound to histones differs from that for the free polymer; the difference spectrum is similar to those for native and poly (dA-dT) . poly (dA-dT) core particles. Under the first two conditions, the 31P NMR spectrum is symmetric with line widths of 91 and 41 Hz, respectively, at 109.3 MHz. In high salt, two 31P peaks of equal intensity are observed, confirming recent results of Patel et al. (1) and indicating an alternating geometry for the phosphodiester backbone. Using this highly homogeneous DNA, we confirm that the Pohl-Jovin transition (2) is an intramolecular rearrangement, not requiring complete strand separation.