A DNA-repair mutant was characterized that has the extraordinary and interesting properties of extreme sensitivity to UV killing combined with a high level of nucleotide excision repair. The mutant V-H1 isolated from the V79 Chinese hamster cell line appeared very stable, with a reversion frequency of about 3.5 X 10(-7). Genetic complementation analysis indicates that V-H1 belongs to the first complementation group of UV-sensitive Chinese hamster ovary (CHO) mutants described by Thompson et al. (1981). This corresponds with data on cross-sensitivity and mutation induction after UV irradiation published by this group. Surprisingly, the mutant V-H1 shows only slightly reduced (to approximately 70%) unscheduled DNA synthesis (UDS) after UV exposure, while the other two mutants of this complementation group are deficient in UDS after UV. In agreement with the high residual UDS, in V-H1 also the amount of repair replication in response to UV treatment is relatively high (approximately 50%). It has also been shown that the incision step of the nucleotide excision pathway takes place in V-H1 (with a lower rate than observed in wild-type cells), whereas another mutant (UV5) of the same complementation group is deficient in incision. This heterogeneity within the first complementation group indicates that the repair gene of this complementation group may have more than one functionally important domain or that the gene is not involved in the incision per se but is involved in e.g. preferential repair of active genes.