Various 5-substituted pyrimidine 2'-deoxyribosides with anti-herpes activity were investigated for their genotoxic activity. 5-Iodo-2'-deoxycytidine (IDC), 5-(2-chloroethyl)-2'-deoxycytidine (CEDC), 5-(3-chloropropyl)-2'-deoxyuridine (CPDU), (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), 5-ethyl-2'-deoxyuridine (EDU), 2'-deoxyuridine (DU) and 2'-deoxythymidine (DT) were non-mutagenic in Salmonella typh. as well as in V79 Chinese hamster cells, 5-Iodo-2'-deoxyuridine (IDU) was moderately mutagenic and 5-(2-chloroethyl)-2'deoxyuridine (CEDU) was highly mutagenic in V79 cells; neither IDU nor CEDU were mutagenic in the bacterial assay. None of the compounds induced unscheduled DNA synthesis in primary rat hepatocytes. In addition, antimutagenic effects of 2'-deoxyuridines were discovered: in V79 cells, BVDU, EDU, DU and DT prevented the mutagenicity induced by CEDU; in these cells EDU also inhibited the mutagenicity induced by MNNG. In primary rat hepatocytes, IDU and EDU inhibited the induction of unscheduled DNA synthesis induced by MNNG, DMBA or UV-light. The compounds were inactive at inducing differentiation in hematopoietic cells. The significance of these data, particularly with regard to the use of 5-substituted 2'-deoxyuridines in anti-herpes therapy, is discussed.