Cr(III), one of the most potent inorganic carcinogens, induces condensation of DNA into a very compact product at 37 degrees, as shown by electron microscopy. The condensation begins with the appearing of some supercoil structures and complete condensation occurs at relatively low Cr(III) concentrations; for 3 and 30 mM ionic strength they are 4.5 and 45 microM, respectively. Under these conditions, Cr(III) inhibits the interaction between ethidium and DNA as shown by absorption and fluorescence spectra.