The physical properties and binding characteristics of the solubilized transferrin-binding protein from BeWo cells, a human choriocarcinoma cell line, were investigated. The binding protein was isolated from 125I-labelled membranes by solubilization followed by immunoprecipitation with anti-human transferrin in the presence of saturating human transferrin. Gel filtration on acrylamide agarose (AcA-22) at 21 degrees C in the absence of transferrin indicates that the transferrin-binding protein has a Stokes' radius of 4.6 nm. In the presence of transferrin, the Stokes' radius of the transferrin-binding BeWo protein increases to 6.3 nm. Parallel sucrose density centrifugation studies indicate that the BeWo protein has a sedimentation coefficient of 9.4 S in the absence of transferrin and 10.9 S in the presence of transferrin. Relative molecular mass calculations from sedimentation studies in H2O and D2O, using the method of Sadler et al (1979), indicate a relative molecular mass of 204,000 for the solubilized receptor and 354,000 for the receptor in the presence of transferrin.