Undetectable or extremely low levels of circulating immunoreactive parathyroid hormone (PTH) have been reported in human newborns while PTH bioactivity was high. This prompted the hypothesis that the fetal calcemic hormone might be PTH-related protein. The purpose of this study was to measure circulating immunoreactive PTH-related protein in human fetuses and newborns in order to investigate this hypothesis. Parathyroid hormone-related protein (PTHrP(1-86) and intact PTH were measured using two-site immunoradiometric assays in plasma obtained by cordocentesis from 23 fetuses (19-33 weeks of gestation), from 17 newborns at term (38-41 weeks), from their mothers and from 22 normal women of reproductive age. Plasma PTHrP was detectable in all but one of the fetuses and newborns and in all the mothers and the controls. The mean level was similar among fetuses (19-33 weeks) (0.43 +/- 0.18 pmol/l), newborns (0.48 +/- 0.12), mothers (0.48 +/- 0.14) and normal controls (0.46 +/- 0.09). Plasma PTH was found to be significantly higher in fetuses at midgestation (1.0 +/- 0.99 pmol/l) than in the newborns (0.22 +/- 0.21) (p < 0.0025); maternal PTH was significantly higher compared to fetal level at mid-gestation (2.1 +/- 1.0, p < 0.01) as well as at term (2.69 +/- 1.40, p < 0.001). In the control women PTH was 3.07 +/- 1.25 pmol/l. These results showed that plasma amino-terminal PTHrP-(1-86) is detectable during the second half of human fetal life and its level remains unchanged during this period of time, in contrast to changing levels of fetal plasma PTH. The relatively low PTHrP-(1-86) level that we found in the newborns is not responsible for the high PTH-like bioactivity found by some investigators in cord blood at term.