Many cytotoxic agents act by causing DNA damage, and the p53 tumour suppressor gene is known to be involved in the cellular response to DNA damage. Since inactivation of p53 is common in many tumours, we wondered if this would affect the sensitivity of cancer cells to cytotoxic agents. We have shown that this is indeed the case in transformed mouse cell lines with and without a mutated p53 gene; p53 "knockout" mouse fibroblasts, and normal human skin fibroblasts treated with an anti-sense p53 oligonucleotide. In addition, we have demonstrated a correlation between p53 protein expression in human breast cancer specimens and their chemosensitivity. The results show that inactivation or mutation of p53 renders cells more sensitive to those cytotoxic drugs whose primary mechanism of action is DNA damage.