We measured parathyroid hormone-related protein (PTHrP) in the serum of 15 healthy women within three days of beginning breast-feeding and within one hour of completing nursing. A sensitive immunoradiometric assay that measures N-terminal PTHrP containing at least the first 74 amino acids was used. We found normal PTHrP values in all patients. Values one day before and two days after delivery in a hypoparathyroid woman who chose not to breast-feed were also normal. Since N-terminal PTHrP fragments of 1-36 amino acids or more are biologically active, we believe additional studies using sensitive PTHrP assays that measure smaller fragments and investigations performed under other conditions of breast-feeding are necessary before concluding that PTHrP does not have a role in calcium homeostasis in nursing mothers. Furthermore, additional studies in hypoparathyroid nursing mothers should be performed.