Placental protein 14 (PP14) levels were measured in serum samples from non-pregnant and pregnant women, amniotic fluid, cord blood, and extracts of placenta, decidua and fetal membranes. The levels were low (15-40 micrograms/l) in serum of non-pregnant women. In four pregnancies following in-vitro fertilization, the serum PP14 levels started to rise 2-12 days after embryo replacement. In normal pregnancy, the highest serum PP14 concentrations (up to 2200 micrograms/l) were detected between 6 and 12 weeks. After 16 weeks the level decreased and plateaued at 24 weeks to around 200 micrograms/l. In amniotic fluid, the highest PP14 levels (232 mg/l) were found between 12 and 20 weeks, being considerably higher than those in maternal serum throughout pregnancy. In cord blood, the levels were low (15-22 micrograms/l) or undetectable. In early pregnancy decidua, the PP14 content was higher (41-160 mg/g total protein) than in late pregnancy decidua (60-2700 micrograms/g total protein). In amnion and chorion laeve, the PP14 concentration varied from 50 to 750 and 50 to 1000 micrograms/g protein, respectively. Early pregnancy placenta contained 0.25-15 mg/g and late pregnancy placenta 3-430 micrograms/g protein of PP14. These results show that the levels of PP14 in pregnancy serum have a similar profile to hCG, but in contrast to other placental proteins, the amniotic fluid PP14 levels are remarkably high. This may be explained by suggesting that decidua is a source of PP14.