Reactive oxygen species (ROS) are thought to play a role in the aging process as well as in a number of human diseases states. Superoxide dismutase (SOD), an enzyme that scavenges the superoxide anion (O2-) is constitutively expressed in leukocytes and other tissues. When assayed in peripheral blood leukocytes (PBL), constitutive SOD activity shows little variation among individuals of different ages. We have found that significant induction of SOD activity occurs in PBL incubated in vitro with paraquat, an agent known to cause intracellular O2- production. This induction was found to be highly age dependent; lymphocytes from 36 healthy subjects aged 20 to 40 years showed an increase of 85% +/- 10%, versus an increase of only 8% +/- 1% for lymphocytes from 30 healthy subjects aged 65 to 79 years (P less than 10(-4)). Forty subjects, aged 67 to 73 years, who were healthy at the time of assay of leukocyte SOD induction were followed up 5 years later. Nineteen of these subjects had died; all 19 had shown SOD induction of less than 10% (range, 0% to 7%; mean, 2.4%). In contrast, of the 21 survivors (range, 2.5% to 50%; mean, 21%), 12 had shown SOD induction greater than 10%, and 7 had shown SOD induction greater than or equal to 35% (P less than 10(-3)). Thirteen of the 19 deaths were attributable to malignancy or cerebrocardiovascular disease. Preservation of leukocyte SOD inducibility appears to correlate with longevity in elderly individuals and may be of value in predicting resistance to malignancy or fetal cardiovascular events.