Exposed sites onPseudomonas maltophilia(ATC #1637), which bind both human chorionic gonadotropin (hCG) and a native hCG-like ligand with equal high affinity, are described. This high-affinity binding site (Kd = 1.3 X 10-10) binds hCG, but does not bind human luteinizing hormone (hLH), nor related human glycoprotein hormones, thyrotropin, and follicle stimulating hormone. A lower affinity, 2.3 X 10-9, is also described which binds hLH and hCG equally well. This is the first description of a high-affinity binding site in nature, which distinguishes hCG from hLH.