Expression of a parathyroid hormone-like protein (PLP), which is associated with hypercalcemia in malignancy, has recently been localized to normal lactating mammary tissue. We examined the possibility of an extramammary role of PLP by measuring its levels in serum and milk of lactating women. The levels of PLP by radioimmunoassay in serum of lactating and nonlactating women were indistinguishable [4.2 +/- 1.8 and 3.6 +/- 1.0 pg equivalents (eq) of PLP-(1-34) amide per ml, respectively]. As PLP was undetectable in some serum samples, this result does not exclude the possibility that lactation results in a small increase in serum levels of PLP. In contrast, high concentrations of immunoreactive PLP [40,000-75,000 pg eq of PLP-(1-34) amide per ml] and correspondingly high concentrations of bioactive PLP were found in human, rat, and bovine milk. A variety of processed bovine milk products had a PLP content similar to that of fresh bovine milk, whereas infant formulas had lower concentrations, ranging down to undetectable. Although the physiological role of PLP in lactation is unknown, the data establish the presence of PLP in milk and suggest the possibility that PLP may be important in neonatal calcium homeostasis.