Parathyroid hormone-related peptide in lactation and in umbilical cord blood. Parathyroid hormone-related peptide (PTHrP) is expressed in lactating rat mammary glands after suckling, as a result of increases in prolactin rather than suckling per se. In addition, PTHrP produced in the fetal parathyroid glands and placenta may be responsible for stimulation of placental calcium transport. In the current study, we used a radioimmunoassay for human PTHrP to measure levels of the peptide in (1) human breast milk, cow's milk, and two infant formulas; (2) sequential plasma samples in prepartum and postpartum lactating women; (3) women with pathologic hyperprolactinemia; and (4) human umbilical cord blood. In normal subjects, plasma PTHrP levels ranged from less than 2 to 5 pmol/liter. In contrast, human breast milk contained substantially increased levels of immunoreactive PTHrP. Similar elevations were found in cow's milk and in one infant formula. Column chromatography of breast milk demonstrated that PTHrP immunoreactivity included a region of adenylate cyclase stimulating activity, consistent with the presence of biologically active PTHrP. Plasma prepartum PTHrP values did not differ from corresponding postpartum values in lactating women. Women with hyperprolactinemia had a mean plasma PTHrP value in the high-normal range. Umbilical cord blood had considerably suppressed parathyroid hormone values but PTHrP levels that were indistinguishable from those in normal human plasma. Thus, PTHrP is present in high concentrations in breast milk but apparently does not gain access to the maternal circulation in significant amounts. In addition, women with pathologic hyperprolactinemia seem not to have increased levels of circulating PTHrP. 