OBJECTIVE Parathyroid hormone-related protein (PTHrP), initially discovered as the factor responsible for the syndrome of humoral hypercalcaemia of malignancy, has also been found to be expressed in placenta, in pregnant uterus, in the fetus at many locations, and in the lactating mammary gland. This study sought to establish whether PTHrP reaches the maternal circulation when it is expressed in mammary tissue during lactation or in the maternal reproductive tract during gestation. METHODS Blood samples were collected from 53 subjects: 18 pregnant women in all stages of gestation, 19 lactating mothers and 16 non-lactating post-partum controls. METHODS PTHrP was measured using a specific and validated radioimmunoassay. Parathyroid hormone was measured by two-site immunoradiometric assay. Total calcium was measured by atomic absorption spectrophotometry. RESULTS Circulating levels of PTHrP were readily detectable in 12 of 19 nursing mothers (range 2.7-7.8 pmol/l) but in none of the mothers who were bottle feeding. PTHrP was also detected in one of 18 pregnant subjects. Parathyroid hormone concentrations were lower in lactating mothers (2.3 +/- 1.0 pmol/l), than in non-lactating mothers (3.5 +/- 1.2 pmol/l) (P < 0.01). CONCLUSIONS PTHrP reaches the maternal circulation during lactation in amounts which could produce a systemic effect.