Parathyroid hormone-related protein (PTHrP) is a recently described hormone, that was isolated from malignant tumors. It shows many properties of parathyroid hormone (PTH) and is related to the pathogenesis of humoral hypercalcemia of malignancy. Therefore, we analyzed PTHrP in the sera of 30 patients with hypercalcemia of malignancy and compared the values with those obtained in patients with primary hyperparathyroidism, Paget's disease of bone, and normal subjects. PTHrP was quantitated with radioimmunoassay (RIA) using aminoterminal antibodies without and with chromatographical sample purification applying SEP-PAK C18 cartridges. Measurements of PTHrP without sample purification yielded high values in all patient groups. There was no differentiation between patient groups. However, quantitation of PTHrP after SEP-PAK C18 purification of the samples resulted in values above the normal range only in tumour patients. In 30 normal subjects PTHrP levels were 110 +/- 75 pg-eq/ml. Eight out of 30 patients with malignant tumours displayed PTHrP-concentrations above 335 pg-eq/ml. PTHrP levels in patients with primary hyperparathyroidism or Paget's disease of bone were within the normal range. PTHrP concentrations were not affected from renal function. We conclude, that determination of PTHrP after sample purification may contribute to the differential diagnosis of malignant disease.