(E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU)is a 5-substituted 2'-deoxyuridine antiviral compound that inhibits thymidylate synthetase. The selectivity of BVDU for virus-infected cells has been attributed to phosphorylation of BVDU by a virus-induced thymidine kinase. Since the closely related compounds 5-bromo-2'-deoxyuridine and 5-iodo-2'-deoxyuridine are in vitro and in vivo mutagens, BVDU was tested for genotoxic activity in bacterial and mammalian cell mutation assays as well as in assays measuring DNA damage/repair and clastogenic activity. Mutation assays with BVDU at concentrations ranging from 10 to 5000 micrograms/plate using Salmonella typhimurium strains TA1535, TA1537, TA1538, TA98, and TA100 were negative, both with and without S9 activation. BVDU was also negative in the in vitro rat hepatocyte unscheduled DNA synthesis assay at concentrations of 750 and 1000 micrograms/ml. In contrast, BVDU was positive in the L5178Y TK +/- mouse lymphoma mutation assay without S9 activation at five concentrations ranging from 500 to 2000 micrograms/ml. A Chinese hamster ovary cell (CHO)/hypoxanthine guanine phosphoribosyl transferase gene mutation assay conducted without S9 over similar concentrations was negative. However, micronucleus induction by BVDU was detected without S9 activation at concentrations between 500 and 1750 micrograms/ml using both CHO and L5178Y cells. These results indicate that BVDU is a potential human clastogen.