Abstract—A plaque assay for adenovirus 2 on normal human fibroblasts has been developed and used to measure the survival of ultraviolet‐irradiated virus on six human fibroblast cell lines. When four xeroderma pigmentosum cell lines were used as viral hosts, an average of one lethal event per virus in the viral population was made with 10, 15, 62, and 78 J m‐2respectively, while using two normal cell lines as hosts, 197 and 205 J m‐2were required to inflict the same damage. These differences are attributed to the known repair deficiency of xeroderma pigmentosum cells, and are discussed in the light of previous data obtained using other animal