The proteins binding retinol, and retinol concentrations, were determined in amniotic fluid, placental cytosol and in the fetal and maternal circulation. 2. In non-pregnant women, plasma retinol was almost exclusively found in a transthyretin-retinol-binding-protein complex whereas, in pregnant women, retinol-binding-protein-bound retinol was observed not complexed to transthyretin. This latter fraction increased in concentration with fetal age. These two fractions were the major retinol-protein complexes in amniotic fluid and their relative amounts changed with progress of gestation. 3. In fetal blood both of these fractions were again found, with higher proportions of retinol-binding-protein-bound retinol in the umbilical artery than in the umbilical vein.
