Therapeutic approaches which are effective in tumour cells resistant to conventional chemotherapy would be of value. An E1B 55 kDa-deleted adenovirus (ONYX-015) induces lysis in cells with mutant p53, although the specificity of these observations for different cell types is unclear. We have used a matched set of drug-resistant human ovarian tumour cell lines to examine the potential of ONYX-015 for preferential replication and lysis of drug-resistant ovarian tumour cells with documented alterations in p53 function. Marked preferential replication of ONYX-015 is observed after infection of mutant p53 transfectant and cisplatin-resistant derivatives, compared to the wild-type p53 expressing parental A2780 line. Infection causes increased cytopathic