The concentration of human lactoferrin (LF) was measured by radioimmunoassay or non-competitive avidin--biotin assay in amniotic fluid, cord blood and in the decidua, trophoblast, fetal membranes and umbilical cord. Amniotic fluid was obtained by amniocentesis, and cord blood and tissue samples were taken after delivery or elective Caesarean section. No detectable concentration of LF was found in amniotic fluid before week 20 of pregnancy. A significant increase in the LF concentration was observed around week 30 and it remained high until term. In cord blood, an undetectable or low concentration of LF was measured. In tissue specimens the amount of LF was highest in the decidua (9-95 micrograms/g), a moderate concentration was assayed in the amniotic (2-37 micrograms/) and chorion (2-26 micrograms/g) membrane and in the trophoblast (5-35 micrograms/g). In the umbilical cord, the concentration was less than 1 microgram/g. These results suggest a decidual origin of LF. The role of LF during pregnancy is discussed.