Patients with the variant form of xeroderma pigmentosum (XP) have clinical XP including a high frequency of skin cancer but, in contrast to the other forms of XP, have normal post-ultraviolet (UV) DNA excision repair and nearly normal post-UV survival. However, like excision repair-deficient XP cells, the XP variant cells are UV hypermutable. We used a UV-treated plasmid shuttle vector, pZ189, to examine the DNA repair defect in lymphoblastoid cells from an XP variant patient, XPPHBE, and a normal control. Plasmid repair, mutagenesis, and replication occur within transfected cells in a process dependent on the cells' repair capacity. With the XP variant cells post-UV, plasmid survival was normal with but there was an abnormally increased post-UV plasmid mutation frequency. Sequence analysis of the mutated plasmids revealed an increased frequency of plasmids with single base substitution mutations with the XP variant cells. As in earlier studies with UV mutagenesis, there was a predominance of G:C-->A:T base substitution mutations with plasmids recovered from both cell lines. The frequency of G:C-->C:G transversions was significantly higher with plasmids recovered from the XP variant cells than from normal cells. The location of mutations in the marker gene was non-random with different mutagenic hotspots found in plasmids recovered from the XP variant cells and from the normal cells. This study suggests that plasmid UV hypermutability in the presence of normal UV survival may be related to the increased UV skin cancer susceptibility of XP variant patients.