Humoral hypercalcemia of malignancy (HHM) most commonly results from secretion by tumors of an unidentified circulating calcemic factor that appears in clinical studies to stimulate both a parathyroid hormone (PTH)-sensitive proximal tubular adenylate cyclase and a distinct PTH-sensitive renal tubular glucose-6-phosphate dehydrogenase complex. In the present study, 8 M urea extracts of tumors from patients with HHM.have been shown to contain both in vitro adenylate cyclase-stimulating activity and glucose-6-phosphate dehydrogenase-stimulating activity as detected in a sensitive cytochemical bioassay. Both the adenylate cyclase-stimulating activity and cytochemical bioactivity are due to specific binding of a substance in the tumor extracts to renal PTH receptors, as demonstrated by competitive inhibition studies using the bovine PTH fragment analogue [Nle8"8, Tyr4]bPTH-(3-34) amides Preincubation with an antiserum to PTH results in no loss of activity in the tumor extract, and the activity appears both on gel filtration and ultrafiltration to be far larger than PTH (estimated Mr 70,000). These studies demonstrate that extracts of tumors from patients with HHM contain a substance that binids to the PTH receptors in the nephron responsible for activation of both the PTH-sensitive glucose-6-phosphate dehydrogenase and the PTH-sensitive adenylate cyclase. This substance is chromatographically and immunologically distinct from PTH. Its role in the genesis of HHM requires further study.Humoral hypercalcemia of malignancy (HHM) results from the secretion by tumors remote from bone of a circulating, calcemic factor (or factors) that stimulates bone resorption (1, 2). The nature of the factor or factors responsible for this humorally mediated hypercalcemia remains elusive. Although vitamin D metabolites, prostaglandins of the E series, and parathyroid hormone (PTH) have in the past been suggested as causative agents (1-6), the available evidence suggests that these agents can account for at best a minority of instances of HHM (1, 2, 7-9). Thus, an as yet unidentified humoral substance appears to be'responsible for the majority of instances of this syndrome (10).Of the calcemic hormones listed above, the humoral factor responsible for HHM most closely resembles PTH. Both patients with HHM and those with primary hyperparathyroidism, the prototype of humorally mediated hypercalcemia, display (i) increased osteoclastic activity in bone (11), (ii) increased nephrogenous cAMP excretion (2, 12, 13), and (iii) increased plasma cytochemical bioactivity in a PTH-sensitive cytochemical bioassay (14). However, there are also a number ofdistinctive points of pathophysiological difference between these two humoral syndromes, in that, unlike primary hyperparathroidism, HHM is associated with: (i) markedly increased fractional calcium-excretion (2, 3), (ii) strikingly decreased plasma concentrations of 1,25-dihydroxyvitamin. D (2), (iii) decreased to absent osteoblastic activity in bone (11), (iv) low to undetectable circulatin...