Indications of an important physiological role of parathyroid hormone-related peptide (PTHrP) for fetal calcium homeostasis, maternal–fetal calcium transport and reproduction have accumulated over recent years. The PTHrP concentrations were measured by an earlier developed midregion radioimmunoassay in serum from lactating healthy females and umbilical cord blood and compared with levels in age-matched non-pregnant or lactating females. The PTHrP concentrations could be measured in all samples after silica cartridge C₁₈ extraction of 10–12 ml of serum. The concentrations were significantly higher during lactation (mean ± sd: 0.72 ± 0.14 pmol/l, N = 22) and in umbilical cord blood (0.85 ± 0.18 pmol/l, N =12) compared with healthy age-matched women (0.48 ± 0.09 pmol/l, N = 10, p< 0.001). The molecular forms of PTHrP were also studied in an agematched control group, in pregnant women and in umbilical cord blood by gel chromatography in a fast protein liquid chromatography system of Sep-Pak-extracted pooled serum. In all three groups we found heterogeneity of the molecular forms with two predominant peaks. The smallest fragment had a molecular weight of 4–6 kD while the largest form appeared as a high-molecular-weight molecule. In conclusion, the concentrations of midmolecule PTHrP fragments in serum are elevated during lactation and in umbilical cord blood. Because the midregion of PTHrP has unique actions, our results indicate that PTHrP may play an important physiological role for the mother and for the maternal–fetal calcium transport.