Ectopic production of isolated alpha subunit of glycoprotein hormones(s) in 32-year-old woman with metastatic lung tumor was first detected by a discordance in the immunological pregnancy tests for native human chorionic gonadotropin (hCG). Quantitative discrepancy of titrations among different types of pregnancy test kit, that i, competitive (hemagglutination or latex agglutination inhibition) and non-competitive (hemagglutination or latex agglutination) reactions, was also confirmed by the experimental study using highly purified ectopic alpha-subunit from the patient's urine. Thus, the discrepancy among the two types of assay systems was due by crossreaction of high concentrations (20,000 ng/ml) of ectopically secreted alpha-subunit of glycoprotein hormone(s). We report herein the typical behavior of immunological assay systems for native hCG in determining immunoreactive free hCG-alpha and stress the diagnostic value of the discordance observed in these assay systems.