• Chromosomes of peripheral leukocytes were examined in 28 addicts participating in a Veterans Administration-Special Action Office for Drug Abuse Prevention (SAODAP) cooperative study of methadyl acetate vs methadone. Blood samples for 72-hour leukocyte cultures were drawn after nearly 40 weeks of maintenance therapy while subjects were receiving active medication. For comparison, ten nondrug users were also studied. The frequency of chromosome damage was not greater in subjects maintained on methadyl acetate or methadone than in nondrug users.