In this study the frequencies of sister-chromatid exchanges (SCEs) were correlated with measurements of the release of reactive oxidants by phagocytes, as determined by luminol-enhanced chemiluminescence (LECL), and levels of the anti-oxidants ascorbate, beta-carotene and vitamin E in blood specimens taken from 65 young asymptomatic cigarette smokers. Increased SCE frequencies correlated with LECL responses (p less than 0.0075) of activated blood phagocytes. Anti-oxidant levels did not correlate with either LECL or SCEs. These findings indicate that increased generation of reactive oxidants by circulating phagocytes from cigarette smokers are associated with cytogenetic changes.