The toxic and mutagenic activities of five antiherpesvirus agents to HeLa cells and herpes simplex virus type 1 (HSV-1) were investigated. 5-Iodo-2′-deoxyuridine (IDU) and 9ß- d-arabinofuranosyladenine (araA) showed very potent inhibitory effects on cell growth and the cloning efficiency of HeLa cells, whereas 1-ß- d-arabinofuranosyl -E-5(2- bromovinyl)uracil (BV-araU), E-5-(2-bromovinyl)-2′-deoxyuridine (BVDU) and 9-(2-hydroxyethoxymethyl)guanine (ACV) showed less inhibitory effect. 50% inhibitory doses of BV-araU and BVDU for cell growth were 657 and 253 μg/ml, respectively. Although the growth inhibitory activity of BVDU was very weak, as above, the mutagenic activity of this drug to the cells, estimated by induction of colchicine-resistant mutants, was observed to be 4 μg/ml, which was a markedly smaller dose than the inhibitory dose for cell growth, and the highest frequency of mutation of cells was shown at 100 μg/ml of BVDU. This activity was more potent than that of IDU. No mutagenic activity of BV-araU, araA and ACV to cells was observed within the concentration range of 1–800 μg/ml. IDU showed high mutagenic activity to HSV-1 growing in human embryo lung fibroblasts, and IDU-resistant mutants were induced at a high frequency. BVDU also induced a small amount of BVDU-resistant mutant virus, although this drug induced many mutant cells. No mutagenic activity of BV-araU, araA and ACV to HSV-1 was observed.