We have investigated the effects of ar-turmerone isolated from turmeric (Curcuma longa L) on DNA of human leukemia cell lines, Molt 4B, HL-60 and stomach cancer KATO III cells. It was found that selective induction of apoptosis by ar-turmerone was observed in human leukemia Molt 4B and HL-60 cells, but not in human stomach cancer KATO III cells. Morphological changes showing apoptotic bodies were observed in the human HL-60 and Molt 4B cells treated with ar-turmerone. The fragmentation of DNA by ar-turmerone to oligonucleosomal-sized fragments that is a characteristic of apoptosis was observed to be concentration- and time-dependent in Molt 4B and HL-60 cells, but not in KATO III cells. The data of the present study show that the suppression by ar-turmerone of growth of these leukemia cell lines results from the induction of apoptosis by this compound.