We have examined circulating concentrations of a parathyroid hormone-like peptide (PLP) in patients with malignancies and in patients with hyperparathyroidism. The radioimmunoassay employed reacts with synthetic amino-terminal fragments of PLP but not with parathyroid hormone. Elevated plasma PLP concentrations were observed in 50% of patients with malignancy and hypercalcemia and in 15% of normocalcemic cancer patients, mean values being higher in the former group. Detectable plasma PLP concentrations were found in 2 of 39 control subjects. In 2 patients with breast cancer plasma PLP declined concomitantly with a reduction in tumor burden. Adenocarcinoma of the breast and squamous cell carcinomas were most frequently associated with high plasma PLP levels although a variety of histologic types were represented. The presence of metastases on bone scans did not correlate with either the severity of hypercalcemia or the extent of PLP elevation. Increased concentrations of plasma PLP were also observed in 4 of 20 patients with primary hyperparathyroidism and in 5 of 16 patients with chronic renal failure and secondary hyperparathyroidism. Gel filtration analysis of immunoreactive PLP in plasma from 2 hypercalcemic breast cancer patients revealed heterogeneity, with, in each case, both large (greater than 15 kD) and small (6-7 kD) molecular weight amino-terminal moieties. The results document the presence of PLP in the circulation of patients with cancer and are consistent with a pathogenetic role for PLP in the hypercalcemia of malignancy irrespective of whether skeletal metastases have occurred. PLP may also contribute to the skeletal and/or renal manifestations of hyperparathyroid states.