We demonstrate the expression of angiotensin II type 1 (AT1) receptors in normal and diseased human breast tissues. Using monoclonal antibody 6313/G2, directed against a specific sequence in the extracellular domain of the AT1 receptor, immunocytochemical analysis revealed positive immunoreactivity in membrane and cytoplasm of specific cell types. Immunoblotting of solubilized proteins separated by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) from benign and malignant tumours identified a single immunoreactive species with a molecular mass of approximately 60 kDa, consistent with that of the mature glycosylated receptor. In studies of [125I]angiotensin II binding using breast membrane preparations, concentrations of specific angiotensin II binding sites were found to range from 1.8 to 100 fmol mg(-1) protein, with a K(d) of approximately 60 nM. Most of the specifically bound [125I]angiotensin II was displaced by losartan, a specific angiotensin II type 1 receptor antagonist, while less was displaced by the AT2 receptor type antagonist, CGP42112A, thus confirming the prevalence of AT1 receptors in this tissue type. These data suggest that the renin-angiotensin system may be involved in normal and abnormal breast tissue function.