The usefulness of serum transferrin receptor (sTfR) as a marker of iron deficiency is limited by lack of standardization of commercial immunoassays for sTfR. An international collaborative study was performed to evaluate a lyophilized preparation of recombinant soluble transferrin receptor (rsTfR) for its suitability to serve as a World Health Organization (WHO) Reference Reagent to standardize immunoassays for sTfR. The concentration of pure rsTfR was determined from the A(280 nm) using the adjusted theoretical extinction coefficient and molecular weight calculated from its sequence, before dilution and lyophilization in a sTfR-depleted serum matrix. Six manufacturers and a health protection laboratory assayed the candidate Reference Reagent, coded 07/202, along with three lyophilized serum samples, using commercial assays for sTfR. Dose-response plots demonstrated acceptable overall parallelism between 07/202, manufacturers' in-house standards, and serum samples. However, there was poor agreement on the estimated (r)sTfR content of 07/202 and serum samples. Expressing the sTfR content of the serum samples relative to 07/202 markedly improved agreement between methods. Use of 07/202 would reduce inter-method variability. The preparation was established as the 1st WHO Reference Reagent for sTfR with assigned free rsTfR monomer values of 21.7 mg/L and 303 nmol/L (0.5 mL reconstitution).