BRCA1 is implicated in cellular responses to DNA damage, thereby substantially contributing to maintenance of the genome integrity. Mutations in the BRCA1 gene occur in breast and ovarian cancer and mutations that disable p53 are frequently found in human cancers, often accompanied by mutations in additional genes, contributing to tumor progression or high-grade malignancy. Therefore, the role of BRCA1 in the sensitivity to anticancer agents in p53-deficient cells was investigated using p53-deficient mouse knockout cell lines either deficient or proficient in Brca1 function. We report that Brca1-deficiency in p53-null cells was associated with increased sensitivity to the topoisomerase I poisons camptothecin and topotecan, the topoisomerase II poisons doxorubicin, mitoxantrone and etoposide, and to the platinum compounds carboplatin and oxaliplatin, but not to the antimetabolites 5-fluorouracil and gemcitabine and the taxanes docetaxel and paclitaxel. The increased growth inhibition to doxorubicin after loss of Brca1 correlated with increased cell killing caused by increased apoptosis. The data presented here indicate that Brca1 modulates p53-independent DNA damage response pathways and they support the case of a role of Brca1 to protect cells from apoptosis-mediated cell death in p53-deficient cells. These results suggest a higher chemotherapy susceptibility of cells disabled in both functions and they foster the concept that functional inhibition of BRCA1 may be a valuable adjunct to anticancer agents to increase the efficacy of chemotherapy in the treatment of p53-mutated cancers.