Premature chromosome condensation of G1 G2, and S-phase chromosomes has been achieved by the use of electrofusion in the fusion of Chinese hamster ovary (CHO) cells and HeLa cells and CHO cells with human leukocytes. Very high yields of heterokaryons, of over 80%, as well as elimination of adverse effects of chemical and viral fusion agents, facilitated induction of premature chromosome condensation of high quality.