Germ-line mutations in BRCA2 have been linked to early-onset familial breast cancer. BRCA2 is known to play a key role in repairing double-strand breaks. Here, we describe the involvement of BRCA2 in O6-alkylguanine DNA alkyltransferase (AGT)-mediated repair of O6-methylguanine adducts. We show that BRCA2 physically associates and undergoes repair-mediated degradation with AGT. In contrast, BRCA2 with a 29-amino-acid deletion in an evolutionarily conserved domain does not bind to alkylated AGT; the two proteins are not degraded; and mouse embryonic fibroblasts are specifically sensitive to alkylating agents that result in O6-methylguanine adducts. We show that O6-benzylguanine (O6BG), a nontoxic inhibitor of AGT, can also induce BRCA2 degradation. BRCA2 is a viable target for cancer therapy because BRCA2-deficient cells are hypersensitive to chemotherapeutic DNA-damaging agents. We show a marked effect of O6BG pretreatment on cell sensitivity to cisplatin. We also show the efficacy of this approach on a wide range of human tumor cell lines, which suggests that chemosensitization of tumors by targeted degradation of BRCA2 may be an important consideration when devising cancer therapeutics.