Peripheral lymphocytes from Taiwanese women (n = 35) exposed to polychlorinated aromatic hydrocarbons and from matched controls (n = 24) were assessed for the levels of sister chromatid exchanges (SCEs) after a 72-hour incubation of whole blood in the presence or absence of alpha-naphthoflavone (ANF) and for chromosome aberrations after 48 hours of incubation. Serum levels of polychlorinated biphenyl (PCB) congeners were measured for all individuals, and serum levels of several polychlorinated dibenzofurans (PCDFs) were measured for 12 exposed individuals by gas chromatography-mass spectometry. Blood concentrations of total PCBs in the exposed population averaged approximately 15 ppb, whereas mean PCDF values were 14 ppt. Major PCB congeners detected were 2,2' 4,4', 5,5'-hexa CB and 2,2'3,4,4',5-hexa CB. PCDFs detected were primarily 1,2,3,4,7,8,-hexachlorodibenzofuran (10.8 ppt) and 2,3,4,7,8-pentachlorodibenzofuran (2.7 ppt). Average SCE frequencies were 7.61 for controls and 7.30 for exposed individuals when assays were conducted in the absence of ANF, whereas respective values were 8.85 and 10.75 in the presence of ANF. Differences in the level of ANF-induced SCEs between the two populations were highly significant (P less than .001). Moreover, the ANF-induced SCEs were highly correlated with the serum concentrations of total PCBs and of several PCB congeners (P less than .001). Increases in ANF-induced SCEs appeared to be linear up to a PCB concentration of approximately 30 ppb. Chromosome aberration frequencies were similar in control and exposed populations. These studies demonstrate that in vivo exposure to PCBs and PCDFs result in an enhanced sensitivity of lymphocytes to the SCE-causing actions of ANF.