Uteroferrin, a purple-colored, iron-containing phosphatase which is induced by progesterone in the porcine uterus, has been proposed to be an intermediary in iron transfer between the mother and conceptus in the pig. Along with a number of other uterine proteins of maternal origin, it accumulates in the allantoic fluid during mid-pregnancy. When [59Fe]uteroferrin was introduced into the allantoic sacs of five fetuses at Day 60 of pregnancy, its iron was transferred to another protein, identified as transferrin. The half-life of iron loss from uteroferrin was approximately 24 h and the kinetics suggested an approximately second order process. 59Fe was also distributed to the fetus and was recovered at high specific activity in the fetal red blood cells (as hemoglobin), liver, and spleen. Introduction of [59Fe] transferrin to allantoic fluid resulted in an almost identical distribution of isotope in the fetus as was observed with [59Fe]uteroferrin. Whereas [125I]transferrin has been shown to be capable of leaving the allantoic sac in intact form, [125I]uteroferrin either has only a limited capacity to exit or else is almost immediately degraded upon entering the fetal circulation. Following loss of its iron to transferrin in allantoic fluid, uteroferrin rapidly loses immunological reactivity and is cleaved proteolytically into small peptides. The transfer of iron between uteroferrin and transferrin has also been followed in vitro. Iron on uteroferrin is relatively stable to pH and is not readily lost to transferrin at around neutral pH unless low molecular iron chelators such as citrate, pyrophosphate, ATP, or ascorbate are present. Iron transfer was promoted at ascorbate concentrations (approximately 50 microM) and at pH values (approximately 6.8) approximating those found in allantoic fluid. Results are consistent with a model in which maternal uteroferrin can transfer its iron to fetal transferrin via a low molecular weight intermediary in the allantoic sac. Transferrin is then responsible for iron distribution to the fetus.