Ultraviolet B (UVB) (290-320 nm) is capable of damaging the DNA molecule directly by generating predominantly pyrimidine dimers. UVA (320-400 nm) does not alter the DNA molecule directly. However, when it is absorbed by cellular photosensitizers, it can damage the DNA molecule indirectly, e.g., by mediation of singlet oxygen, generating predominantly 8-hydroxyguanine. These indirect effects have been implicated in the mutagenic, genotoxic, and carcinogenic effects of UVA. To study the processing of directly and indirectly UV-induced DNA damage in intact, DNA-repair-proficient and -deficient human cells, we used the replicating plasmid pRSVcat, either irradiated with up to 10 kJ/m2 UVB or treated with the photosensitizer methylene blue plus visible light (which generates singlet oxygen). These treated plasmids were introduced into lymphoblast lines from normal donors or from patients with xeroderma pigmentosum (XP) complementation groups A, C, D, E, and variant. DNA repair was assessed by measuring activity of reactivated chloramphenicol-acetyl-transferase enzyme, encoded by the plasmid's cat gene, in cell extracts after 3 d. As expected, the repair of UVB-induced DNA damage was reduced in all XP cell lines, and the degree varied with the complementation group. XP-A, -D, -E, and -variant cells were normally efficient in the repair of singlet oxygen-induced DNA damage. Only three of four XP-C cell lines showed a markedly reduced repair of these lesions. This indicates differential DNA-repair pathways for directly and indirectly UV-induced DNA damage in human cells and suggests that both may be affected in XP-C.