Poly(dA-dC).poly(dG-dT) was studied by circular dichroism in the presence of high CsCl concentrations and in ethanolic solutions. This alternating purine-pyrimidine duplex may undergo two conformational transitions from a B-type to a novel structure and subsequently into an A-form. Cs+ ions or increasing ethanol concentrations induced a change of the B-type CD spectrum and an inversion of the long wavelength CD band. Lowering the temperature below 0 C or addition of small amounts of Ca++ ions were particularly potent in producing a large negative CD band. A modified B-type structure or a conversion into a left-handed Z-form is considered for this conformational transition.