Transferrin was identified in a preparation of human syncytiotrophoblast basal plasma membrane. Such transferrin was shown to be bound to an amphiphilic membrane protein of subunit mol.wt 94 000 by crossed hydrophobic interaction immunoelectrophoresis and immunoprecipitation with antibodies to transferrin. Basal plasma membrane bound 125I-labelled transferrin in a saturable, reversible manner with high affinity (Kd = 2.5 +/- 0.6 X 10(-9) M). The maximum binding capacity (0.9 +/- 0.2 pmol transferrin/mg membrane protein) was approximately a half of that of microvillous membrane. The basal membrane transferrin receptors were similar to microvillous receptors in that their affinity for diferric transferrin was higher than that for apo-transferrin and transferrin dissociation was negligible at pH 5.0 but rapid at pH 7.4. We conclude that syncytiotrophoblast basal plasma membrane possesses a receptor similar, if not identical, to that on the microvillous membrane. These receptors are thus in a position to participate in iron transfer to the fetus or potentially to have alternative functions in the syncytiotrophoblast.