The exfoliated cell micronucleus (MN) assay using fluorescent in situ hybridization (FISH) with a centromeric probe is a rapid method for determining the mechanism of MN formation in epithelial tissues exposed to carcinogenic agents. Here, we describe the use of this assay to detect the presence or absence of centromeric DNA in MN induced in vivo by radiation therapy and chronic arsenic (As) ingestion. We examined the buccal cells of an individual receiving 6,500 rods of photon radiation to the head and neck. Exfoliated cells were collected before, during, and after treatment. After radiation exposure a 16.6-fold increase in buccal cell MN frequency was seen. All induced MN were centrcmere negative (MN-) resulting from chromosome breakage. This finding is consistent with the clastcgenic action of radiation and confirmed the reliability of the method. Three weeks post-therapy, MN frequencies returned to baseline. We also applied the assay to exfoliated bladder cells of 18 people chronically exposed to high levels of inorganic arse-nic (In-As) in drinking water (average level, 1,3 12 Fg As/L) and 18 matched controls (average level, 16 pg As/L). The combined increase in MN fre quency was 1.8-fold (P = 0.001, Fisher's exact test). Frequencies of micronuclei containing acentric fragments (MN-) and those containing whole chromosomes (MN+) both increased (1.65-fold, P = 0.07, and 1.37-fold, P = 0.15, respectively), suggesting that arsenic may have both clastogenic and weak aneuploidogenic properties in vivo. After stratification on sex, the effect was stronger in male than in female bladder cells. In males the MNfrequency increased 2.06-fold (P = 0.07) while the frequency of MN+ increased 1.86-fold (P = 0.08). In addition, the frequencies of MNand MN+ were positively associated with urinary arsenic and its metabolites. However, the association was stronger for micronuclei containing acentric fragments. By using FISH with centromeric probes, the mechanism of chem ically induced genotoxicity can now be determined in epithelial tissues.