Breast cancer vasoactive intestinal peptide (VIP) receptors were characterized. Using in vitro autoradiographic techniques, 125I-labeled VIP bound with high affinity to breast biopsy sections. 125I-labeled VIP bound specifically to five breast cancer cell lines examined using receptor- binding techniques. Specific 125I-labeled VIP binding to MDA-MB-231 cells was inhibited with high affinity by VIP and pituitary adenylate cyclase- activating polypeptide (IC50 = 2 nM) and with moderate affinity by the VIP hybrid (IC50 = 0.5 μM). VIP elevated the cAMP in a dose-dependent manner, and VIP hybrid (10 μM) inhibited the increase in cAMP caused by VIP. Using Northern blot analysis, VIP (10 nM) stimulated c-fos and c-myc mRNA, and the increase caused by VIP was reversed by the VIP hybrid. The VIP hybrid inhibited breast cancer growth in vitro and in vivo using nude mice bearing breast cancer xenografts. These data suggest that the VIP hybrid is a breast cancer VIP receptor antagonist.