In Korea, all domestic made test systems for detecting antibodies in HIV-1 contain the antigens from human immunodeficiency type 1 (HIV-1) subtype B. However, because HIV-1 subtype O is significantly different in amino acid sequences from all other subtypes of HIV-1, there has been a need for developing a test for detecting antibodies in subtype O. For this purpose, the entire nucleotide sequence corresponding to the extracellular domain of the transmembrane glycoprotein of HIV-1 subtype O was synthesized with consideration of Escherichia coli condon usage. Various regions of the extracellular domain were cloned into E. coli expression vectors and tested for levels of protein production. The nucleotide sequence, named ECTM, that can encode a 129 amino acid-long peptide, was found to be expressed at a high level in E. coli. The protein of approximately 17 kDa specifically reacted with sera from individuals infected with HIV-1 subtype O. The ECTM protein was purified to near homogeneity by the CM-T gel chromatography, using concentrated, denatured inclusion bodies. In Western blot analysis, the purified viral antigen reacted with sera from individuals infected with subtype O more efficiently than subtype B. The enzyme linked immunoabsorbent assay (ELISA) system was developed using the subtype O viral protein and compared with the commercially available kit lacking the antigens from subtype O. The ELISA kit containing the subtype O antigen ECTM alone efficiently reacted with sera from individuals infected with subtype O. The subtype O antigen-containing kit produced a positive absorbence even when sera were diluted 512-fold, suggesting a high sensitivity. The commercially available kit also reacted with subtype O sera, but produced a negative result at a dilution of 8-fold. Our results suggest that the currently available kit may not be able to efficiently detect subtype O sera and that the viral protein developed in this study may be added to the current system to maximize the detection of sera from individuals infected with subtype O.