Parathyroid-hormone-related protein (PTHrP) is found in high concentrations in human milk. One of the possible physiological roles of PTHrP may be the regulation of calcium transport through the mammary gland. The relationship between milk PTHrP and Ca concentrations was evaluated in lactating women in the present study. Milk PTHrP has been quantified by immunoradiometric assay (IRMA) and radioimmunoassay (RIA). The IRMA detects PTHrP 1–72, and the RIA carboxyterminal region of PTHrP (C-PTHrP). Milk PTHrP 1–72 and C-PTHrP concentrations did not significantly change by breast-feeding. Milk PTHrP 1–72 changed significantly by the time of day, in contrast to C-PTHrP. Thus, PTHrP 1–72 rather than C-PTHrP may reflect an acute change in mammary PTHrP secretion. In lactating women in whom milk PTHrP and calcium concentrations were serially measured at 5-day intervals up to 26–30 days after delivery, milk PTHrP 1–72 progressively increased with duration of lactation. In contrast to PTHrP 1–72, milk C-PTHrP decreased during this period. And milk PTHrP 1–72 levels inversely correlated with C-PTHrP levels. Milk C-PTHrP but not PTHrP 1–72 levels correlated significantly with milk Ca concentrations. Although it is not certain whether the correlation between milk C-PTHrP and Ca levels is causally related, it may be that fragments of PTHrP detected as C-PTHrP are involved in the regulation of Ca transport through the mammary gland in women.