Exposing (soluble) calf thymus chromatin and, as reference, protein-free native calf thymus DNA (both in 0.01 M Na+, pH 6.8, 25 degrees C) to increasing concentrations of CH3HgOh produces cooperative transitions in their CD spectra. In the case of chromatin, and there especially at low concentrations of methylmercury, they are due to reactions affecting the relative orientation of the bases in the constituent DNA, without disrupting base-pairing. In the case of protein-free DNA, and with chromatin at higher methylmercury concentrations, the CD changes signal collapse of the DNA secondary structure. Primary data (molar ellipticities [theta], zero-ellipticity points, and rotational strengths R) are presented as a function of methylmercury concentration and wavelength. The results are discussed in relation to previous findings of this laboratory regarding methylmercury-DNA and methylmercury-chromatin interactions, and it is pointed out that the structural alterations observed with chromatin at low levels of methylmercury may very well be the primary events in a chain that is responsible for the teratogenic and clastogenic damages caused by organic mercury.