This monoclonal antibody reacts with a protein of 22kD, identified as bet a sub-unit of HCG. It does not cross react with the alpha sub-unit. HCG is a glycoprotein, which is secrete d in large quantities by normal trophoblasts. It is present only in trace amounts in non-pregnant urine and sera but rises sharply during pregnancy. HCG is composed of two non-identical, non-covalently linked polypeptide cha ins designated as the alpha and beta subunits. The alpha subunit is identical to that of thyroid stim ulating hormone (TSH), follicle stimulating hormone (FSH), and luteinizing hormone (LH). hCG monoclonal antibody detects cells and tumors of trophoblastic origin such as choriocarcinoma. Large cell carcinoma and adenocarc inoma of the lung demonstrate anti-hCG positivity in 90% and 60% of cases respectively. 20% of l ung squamous cell carcinomas are positive. hCG expression by non-trophoblastic tumors may indicate aggressive behavior.