Total thyroxine--binding globulin (TBGt) and its pregnancy-associated molecular variant (TBG-1) were detected by radioimmunoassay in human amniotic fluid collected at the time of delivery. TBGt purified from amniotic fluid by affinity chromatography and hydroxylapatite chromatography, displayed electrophoretic properties, immunoreactivity, a molecular mass, affinity for thyroxine and TBG-1 content identical to those of pure TBGt from human pregnancy serum. The concentrations of TBGt and TBG-1 in maternal venous blood serum were 49 +/- 7 mg/ml and 3.8 +/- 1.3 mg/ml (n = 23), respectively, and those in amniotic fluid were 2.0 +/- 0.5 mg/ml and 0.19 +/- 0.12 mg/ml (n = 30), respectively. The analysis of paired samples showed that the serum and amniotic fluid TBGt levels closely correlated (r = 0.91), whereas the correlation between the respective TBG-1 levels was poor (r = 0.63). The TBG-1/TBGt ratio in amniotic fluid was 20% higher than that in serum. These findings suggested a maternal origin of the most of the amniotic fluid TBG which may possibly enter amniotic fluid by passive diffusion through fetal membranes. An alternative mechanism can exist for TBG-1 transfer from maternal serum to amniotic fluid. It corresponds to a special role that TBG-1 may play in the fetoplacental system.