The widespread expression of the gene for PTH-related protein (PTHrP) and the high interspecies conservation of the primary sequence of even the non-PTH-like portion of the protein argue for a vital role(s) for PTHrP in normal physiology. Emerging evidence suggests that PTHrP may be processed into smaller bioactive peptides, but the circulating forms of PTHrP are not well characterized. We have measured plasma concentrations in well defined patient groups using a RIA directed toward midregion PTHrP-(37-74), compared midregion concentrations to amino-terminal and carboxy-terminal PTHrP concentrations in the same patients, and further defined the components of midregion PTHrP immunoreactivity by high pressure liquid chromatography. Patients with humoral hypercalcemia of malignancy (HHM) had concentrations of PTHrP-(37-74) immunoreactivity of 90 +/- 10 pmol/L (mean +/- SEM), 9-fold higher than PTHrP-(1-74) immunoreactivity and about 3-fold higher than PTHrP-(109-138) immunoactivity. There was no consistent elevation of midregion PTHrP in patients with local osteolytic hypercalcemia, hyperparathyroidism, or renal failure, but discrimination of these groups from HHM was less complete using PTHrP-(37-74) than using PTHrP-(1-74) immunoactivity. By reverse phase high pressure liquid chromatography, plasma PTHrP-(37-74) immunoactivity in patients with HHM was resolved into three components: 1) a major peak coeluting with that found in medium conditioned by cells transfected with human PTHrP-(1-141), which we have previously sequenced and found to represent a midregion peptide beginning at residue 38; 2) a minor peak with both PTHrP-(37-74) and -(1-74) immunoreactivity; and 3) another minor peak with PTHrP-(37-74), but not PTHrP-(1-74), immunoactivity. In conclusion, the predominant circulating form of PTHrP in patients with HHM is a midregion species similar or identical to the peptide beginning at residue 38, which has been shown to be a secretory form of PTHrP.