Placental transport of uteroferrin (Uf), the progesterone-induced iron transport glycoprotein, and its distribution within the fetus were investigated by the peroxidase-antiperoxidase bridge (PAP) technique. In Experiment 1, Uf was localized in endometrial and placental tissues taken from gilts on Days 60, 75, 90 and 105 of pregnancy. Uteroferrin was observed within cells of the endometrial glands and surface epithelium adjacent to placental areolae but not in endometrial surface epithelium between areolae. Heavy staining for Uf was observed in cells of the areolae and was associated with both supra- and infranuclear cytoplasmic vesicles. Vesicles located within the infranuclear cytoplasm were occasionally observed to be releasing their contents into capillaries surrounding the areolae. In Experiment 2, Uf was measured by radioimmunoassay in blood samples taken from the umbilical vein and artery of fetuses on Day 75 of pregnancy. Uteroferrin concentrations were greater (P less than 0.07) in umbilical vein blood (79.8 +/- 13.1 ng/ml) than in umbilical artery blood (43.9 +/- 13.1 ng/ml). Uteroferrin binding by Day 75 fetal liver membranes was examined in Experiment 3. Binding of 125I-Uf increased linearly with increasing quantities of membrane protein and binding of 125I-Uf was competitively inhibited by adding unlabeled Uf to the assay. In Experiment 4, urine samples were taken from Day 75 fetuses and assayed for beta-mercaptoethanol activated acid phosphatase activity which is indicative of Uf. In addition, urine proteins were analyzed for Uf by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and Ouchterlony double immunodiffusion (ID). Acid phosphatase specific activity was 3.37 +/- 1.83 mumol substrate hydrolyzed/10 min per mg protein. Uteroferrin was detected in two of three urine samples by 2D-PAGE and in three of eight samples by ID. In Experiment 5, tissue distribution of Uf was determined by the PAP technique in liver and kidney tissue taken from fetuses on Day 75 of pregnancy. Staining for Uf was observed in collecting ducts and proximal tubules of kidney tissue, but staining was not observed in liver tissue. These results indicate that Uf is transported by the areolae into the chorioallantoic capillaries and to the fetus by the umbilical vein. Within the fetus Uf is either bound by the liver, probably to supply iron for hematopoiesis, or cleared by the kidney and transported within the urine to the allantoic sac to serve as a temporary iron storage reservoir.