Three UV sensitive (UV(s)) mutants (CHO43RO, CHO423PV, CHO30PV), characterized by different levels of reduction in their ability to perform unscheduled DNA synthesis (UDS), were analysed for spontaneous and UV-induced frequency of chromosomal aberrations and for sensitivity to alkylating agents. The baseline frequency of chromosomal aberrations was in the normal range, whereas after UV irradiation a positive correlation between the degree of UV sensitivity and the rate of chromosomal breakage was observed. Survival experiments after mutagen exposure indicated that the UV(s) clones are characterized by different levels of hypersensitivity to bifunctional alkylating agents whereas the sensitivity to monofunctional alkylating agents is in the normal range. Genetic analysis performed by measuring the survival after UV in hybrids produced by fusing UV(s) cells with wild-type or UV(s) cells belonging to the six Chinese hamster complementation groups, indicated that the three clones carry recessive mutations and belong to c.g. 2. These findings suggest that defects in the same gene may result in different degrees of phenotypic alterations.