DNA conformations in aqueous solutions containing various dimethyl sulfoxide (DMSO) concentrations were studied. Small (to 7 vol %) DMSO concentrations do not affect the spectroscopic characteristics, volume, or persistent length of a DNA molecule. Higher DMSO concentrations alter the circular dichroism (CD) spectra of the DNA. The optical anisotropy of the macromolecule changes, and the coil volume decreases, but the double-helix DNA structure is not noticeably perturbed. DMSO effects on native and denatured DNA are compared. A