Lymphocyte cultures from the gorilla, chimpanzee, and orangutan were treated with the oligopeptide antibiotic distamycin A. This AT-specific DNA-ligand induces a distinct undercondensation in the quinacrine-bright heterochromatin of the gorilla and chimpanzee. This is also the case in human lymphocyte cultures. Distamycin A further causes an undercondensation in the nonheterochromatic bands 17q21 of the gorilla and 16q22 of man. No visible distamycin A-sensitive chromosome regions are determined in the orangutan. The in vitro treatment with distamycin A preserves the somatic pairings between the quinacrine-bright heterochromatic regions existing in the interphase nucleus until the succeeding metaphase stage. The phylogenetic origin of the quinacrine-bright and distamycin A-sensitive heterochromatin in the ancestor of man, the gorilla, and the chimpanzee is discussed.