Diethylstilbestrol (DES), a synthetic estrogen, showed colchicine-like effects in vitro on cells of the cell lines such as Chinese hamster fibroblast of thymus origin (CHT), rat liver (DL), rat erythroblastic leukemia (EDEN-1/TC) and HeLa-S3. Metaphase arrest was induced 3 h after treatment with 15 microgram/ml of DES and polyploid or polynucleated cells were prominently observed more than 24 h after treatment. The arrest, however, was reversible when the agent was removed from the medium. Tetraploid karotypes induced by DES in CHT cells consisted of all double sets of diploid chromosome constitution except one chromosome marker. By clonal selection, several hypotetraploid sublines were successfully isolated from a CHT cell population after the treatment with 15 microgram/ml of DES for 48 h. Some comparative studies of cytological effects of DES with those induced by colcemid indicated that the DES effect was also a mitotic inhibition similar to colchicine.