The previously developed adenovirus host-cell plaque assay (in which adenovirus 2 or 5 is treated with physical or chemical agents, and is then assayed for biological activity on monolayers of human cells) was used in three studies: 1) Fibroblasts from persons afflicted with Cockayne's syndrome were found less able than fibroblasts from apparently normal persons to repair DNA damage due to ultraviolet light; 2) ultraviolet light was found to cause mutations (reversions) in an adenovirus 5 temperature sensitive mutant; and 3) N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) treatment of human kidney carcinoma cells was found to decrease their capacity to repair MNNG treated adenovirus 5.