Transferrin receptor (TfR) is a dimeric transmembrane protein that provides iron transport from plasma to cells by binding and internalization of iron-loaded transferrin (Tf). Soluble transferrin receptor (sTfR) is an extracellular part of the TfR molecule that is truncated from the cell surface and released into the blood stream. Using monoclonal antibodies (HyTest Ltd., Turku, Finland), immunofluorescent methods for sTfR and sTfR-Tf complex determination were developed. Soluble TfR was isolated from human plasma, and complex formation between sTfR and Tf was studied by stepwise complex construction and by FPLC gel filtration. It was found that sTfR could bind two Tf molecules step by step when the sTfR-Tf complex is constructed in the plate wells. FPLC gel filtration of sTfR-Tf mixtures and analysis of sTfR and sTfR-Tf immunological activities in collected fractions showed that sTfR can form different complexes with TF depending upon the ratios between them: a 291-kDa compound is assumed to be a 2:1 sTfR/Tf complex, and a 345-kDa compound is assumed to be a 2:2 sTfR/Tf complex. Isolated sTfR eluted as a 237-kDa protein. FPLC gel filtration of serum revealed that all sTfR in serum is bound to Tf in a 2:2 complex, and no isolated sTfR can be found in serum. This raises the question as to the nature of the bonds that hold two molecules of sTfR together to form a dimer.