Among various strains of skin fibroblasts tested, two strains derived from xeroderma pigmentosum (XP) patients (ages 19 and 25) with neurological complications and two strains obtained from heterozygotes (ages 54 and 18) showed relatively higher susceptibility than normal age-matched controls to transformation by feline sarcoma virus (FSV). Only one strain from a normal individual also showed a high susceptibility. Generally, there was a parallelism in susceptibilities to FSV and Kirsten murine sarcoma virus (KiMsv). However, cells from normal individuals of 46 years or older exhibited high ratios of FSV:KiMSV titers which were due to their lower susceptibility to KiMSV. Cells from two XP patients (ages 25 and 22) and a heterozygote (age 18), who were in a younger age group, manifested such a differential susceptibility to FSV and KiMSV. There was a correlation between the relative sensitivity of XP cells to the cytotoxic effect of 4-nitroquinoline 1-oxide and killing effect of UV light. Pretreatment of fibroblasts from three XP patients by a subtoxic dose of 4-nitroquinoline 1-oxide 24 hr before viral infection facilitated transformation by KiMSV and FSV, whereas no such effect was observed with three normal cells strains similarly treated.