A range of hamster cell mutants, which have been characterised as sensitive to ionising radiation, were examined for their cross-sensitivity to four DNA-DNA cross-linking agents and the protein-DNA cross-linking agent, camptothecin. The mutants represent 7 distinct complementation groups. Two complementation groups were identified as having a major sensitivity to cross-linking damage, more marked than their sensitivity to ionising radiation (irs1, irs1SF). These two mutants also show sensitivity to UV-irradiation. Two of the remaining complementation groups (xrs and XR-1) have a defect in rejoining DNA double-strand breaks, and these exhibit sensitivity to 3 of the 4 DNA-DNA cross-linking agents. The results with these mutants suggest an involvement of double-strand break rejoining in the repair of certain cross-link damage. Two mutants were also notably sensitive to the topoisomerase I inhibiting anticancer drug, camptothecin. One of these mutants was sensitive to the DNA cross-linking agents examined (irs1SF), but the other was not at all sensitive to this class of drug (EM9).