Trophoblastic beta 1-glycoprotein (TBG) or a "specific protein of pregnancy" isolated from placental extracts and retroplacental serum, is a reliable marker of trophoblastic tumors. A highly sensitive quantitative radioimmunoassay to determine TBG was developed. The use of this method in the Department of Gynecology. All-Union Cancer Research Center, USSR AMS, for a dynamic observation over patients with different trophoblastic tumors for 3 yrs. has shown that TBG is determined in 94.7% of the cases, this antigen concentration correlating with the disease stage and reflecting a clinical course of the disease. Chorionic gonadotropin (CG) determined simultaneously in the urine of the same patients, was found in 84% of the cases. TBG indices correlate with the CG level in 75% of the cases and serve as an important criterion in the diagnosis and evaluation of efficacy of treatment provided to patients with trophoblastic tumors.