E. coli HB101 harboring plasmid pUC19 was grown in the presence of 5-formyl-2'-deoxyuridine (fdU) to evaluate the genotoxic and cytotoxic potentials associated with this DNA lesion. Cell growth was inhibited by fdU in a concentration-dependent manner, but increased mutation was not observed in the lacZ(alpha) gene of pUC19. The lack of the mutagenic effect was attributed to poor utilization of fdU as a substrate by thymidine kinase, which converts exogenous thymidine analogs to the corresponding 5'-monophosphates in the salvage pathway.