Immunohistochemical identification of the PTHR1 parathyroid hormone receptor in normal and neoplastic human tissues. Parathyroid hormone (PTH) is a crucial regulator of calcium homoeostasis in humans. Although it is well known that PTH acts primarily on kidney and bone, the precise cellular and subcellular sites of PTH action have not been visualised in human tissues.We developed and characterised a novel anti-peptide antibody to the carboxy-terminal region of the human PTH receptor type 1 (PTHR1). Specificity of the antiserum was demonstrated by i) detection of a broad band migrating at M(r) 85,000-95,000 in western blots of membranes from human kidney and PTHR1-transfected cells; ii) cell surface staining of PTHR1-transfected cells; iii) translocation of PTHR1 receptor immunostaining after agonist exposure; and iv) abolition of tissue immunostaining by preadsorption of the antibody with its immunising peptide. The distribution of PTHR1 receptors was investigated in 320 human tumours and their tissues of origin.In the kidney, PTHR1 receptors were predominantly detected at the basolateral plasma membrane of epithelial cells in the proximal and distal tubules but not in the thin limbs of Henle, collecting ducts or glomeruli. In bone, PTHR1 receptors were detected as discrete plasma membrane staining of osteocytes and osteoblasts, whereas osteoclasts remained unstained. In addition, PTHR1 was found in the gut and in a number of neoplastic tissues including colorectal carcinoma, prostate cancer, renal cell carcinoma and osteosarcoma.This is the first localisation of PTHR1 receptors in human tissues at the cellular level. The overexpression of PTHR1 receptors may provide a molecular basis for efficient targeting of human tumours with radiolabelled PTH analogues.