Two different assay systems have been developed for the measurement of asialo hCG(ashCG). One of them is an immunoradiometric assay which utilizes peanut lectin (PNA) to selectively extract ashCG and then directly quantify it with purified radiolabeled antiserum which is generated against carboxy terminal peptide of hCG beta (beta-CTP). PNA is highly specific for the terminal carbohydrate linkage Gal beta-(1----3)-Gal NAc which is only exposed to hCG when the O-serine linked oligosaccharide of its unique beta-CTP region is deficient in sialic acid. The other is an improved RIA using a specific antiserum to as beta-CTP, in which extraction of hCG with monoclonal antibody-Sepharose 4B conjugate is used to obtain increased sensitivity with retention of specificity. The ashCG concentration can be measured as low as 0.01 and 0.05 pmoles/ml in the presence of an excess amount of hCG in each of these assays. These assays were applied to detect ashCG in the urine of patients with trophoblastic tumors and normal pregnant women. The proportion of ashCG to total hCG in choriocarcinoma was significantly higher than that in normal pregnancy in both assays. However, the proportion was dependent upon the amount of total hCG and it decreased as total hCG increased.