The low-salt Z form of poly(dm5C-dG) reported in the literature was shown to be due to minute amounts of bivalent cations. If the polymer was purified by dialysis against 1 mM EDTA, poly(dm5C-dG) showed the normal B form. No change in this B form was observed up to 0.8 M NaCl. Very small amounts of Mg2+ or Ca2+ (10(-6) to 10(-5) M) induced the characteristic Z form CD spectrum, which could easily and rapidly be reverted to B-DNA by addition of EDTA.