Parathyroid hormone-related protein (PTHrP), the hypercalcaemia of malignancy factor, is expressed in the tissues of the human uteroplacental unit, including the placenta, amnion and chorion. We have used three region-specific immunoassays to quantitate and compare the distribution of PTHrP in tissues obtained at term following spontaneous labour and vaginal delivery or elective Caesarean section. In non-labouring women highest PTHrP(1-86) and (37-67) immunoreactivity was found in amnion covering the placenta, rather than the decidua parietalis of the uterus (reflected amnion) (median 1020 vs 451 fmol/g; 2181 vs 1444 fmol/g respectively). In labouring women, the PTHrP(1-86) concentration in reflected amnion was inversely correlated with the interval between rupture of the membranes and delivery. Tissue PTHrP(1-86) concentrations were lower in placenta than in chorion and amnion (medians 12, 109 and 664 fmol/g respectively) and, in all tissues, PTHrP(1-34) and (37-67) concentrations were significantly higher than that of PTHrP(1-86). Bioactive PTHrP(1-34) was detected in placenta, chorion and amnion using the ROS cell bioassay. The PTHrP(1-86) concentration (mean +/- S.E.M. = 41.4 +/- 4.5 pmol/l) was high in amniotic fluid at term, although in maternal and cord plasma levels were only modestly increased. The molecular forms of PTHrP present in tissues and amniotic fluid were investigated by column chromatography which confirmed its molecular heterogeneity and suggested that processing is tissue-specific and occurs at both amino- and carboxy-terminals of the peptide.