To investigate the function of the N-terminal immunoglobulin (Ig)-like domain of the human interleukin-6 receptor alpha-chain (hIL-6R), we constructed a soluble human interleukin-6 receptor (shIL-6R) (named EC05, amino acids 20-354) and soluble variants of the shIL-6R lacking the Ig-like domain (named EC70, amino acids 105-354). The two extracellular portions of hIL-6R were expressed as soluble fusion proteins with thioredoxin in Escherichia coli and purified by using Ni-NTA agarose. Western blot showed that purified proteins were immunoreactive with the antibody against hIL-6R. They also possessed specific binding activity with human interleukin-6 (hIL-6) in ELISA analysis.