In contrast to relatively well developed experimental and theoretical studies on polypeptides and proteins (see Gratzer 1967 and McLachlan 1967, this volume) the investigation of optical activity of polynucleotides and nucleic acids were very restricted. The optical rotatory dispersion curves of polynucleotides examined in the visible and near u. v. fit one-term Drude equation regardless of the conformation (Fresco 1961; Levedahl & James 1957; Ts’o, Helmkamp & Sander 1962). Recent circular dichroism (c. d.) measurements of several polynucleotides and nucleic acids (figure 1) indicated clearly the presence of dichroic bands in the u. v. region of base absorption which can be related to the dissymmetrical helical conformation (Brahms 1963). The intensity of circular dichroic bands decreases strongly under the conditions in which the helical structure is unstable and goes to random coil form (Brahms 1964; Brahms & Mommaerts 1964). Thus polyadenylic acid (poly A) is known according to X-ray data to exist at acid pH in a helical two strand and right handed conformation (Rich, Davies, Crick & Watson 1961). In acid solution the same polyadenylic acid exhibits strong circular dichroic bands which disappear at high temperature (figure 2).