The optical rotatory dispersion curves of the proflavine cation were measured in the spectral range 400-500 mp. No optical activity was observed for the free cation but a large positive Cotton effect appeared in the presence of DNA. The effect of ionic. strength, denaturation of the DNA, and the l)NA/proflavine ratio were studied. The dependence of the magnitude of the Cotton effect on the DNA/proflavine ratio suggest.. that a nearest-neighbor interaction between bound proflavine molecules is necessary for optical activity. A simple statistical treatment was made which indicated that only a small number of proflavine molecules are required in close proximity for optical activity to occur. Denaturation of the DNA did not destroy the optical activity, which shows that long runs of DNA double helix are not necessary for optical activity of the ligand molecules. The optical rotatory dispersion curves of acridine orange which was bound to DNA were also measured. Two Cotton effects of opposite sense could be distinguished, the relative magnitudes of which depended on the DNA/acridine orange ratio and the state of denaturation of the DNA. The apparent differences from the proflaviiie-DNA system can to a large extent be explained in terms of the tendency of acridine orange to form aggregates.