Two DNA plasmids, pEGF and pACL29, intercalated with ethidium bromide (EB), have been examined by circular dichroism (CD) and dynamic light scattering (DLS). CD and DLS data show significant changes when the EB/DNA (phosphates) ratio reaches a value of r approximately equal to 0.13. The translational and rotational diffusion coefficients, predicted assuming that plasmids can be described by a string of beads, and the CD spectrum, suggest that a transition from an interwound to a toroidal conformation is likely to occur.