In order to find out whether the serum levels of three placental proteins, human chorionic gonsrtotrpin B-suhunit(B-hCG), Schwangershafts Protein l(SP1) and placental protein l0(PP10), can be uaed for prediction of the response of chemotherapy and differential diagnosis of hydatidiforrn mole invasive mole and chorioearcinoma, we measured the circulating levels of three plaeental protein in 22 patients with hydatidiforrn mole, 25 patients with invasive mole and 25 patients with choriocarcinoma who were registered and treated in Korean Research Institute of Trophoblastic Disease, Catholie University Medical College, Seoul, Korea. Eaeh group of patients ware divided into high risk and low risk groups according to the modified Bagshawes scoring sysiem. PPm a new placental glycoprotein, was measured by a specific and sensitive double antibody radioimmu- noassay in each serum sample. Both SP, and B-hCG are thought to be mainly produced by syncytiotrophoblasts, so we measured the SP1/ B-hCG ratio to eliminate the influence of the number of trophoblastic cells. The results were as follows. 1. The SP1/B-hCG ratio of each group was 18.6+/-12.69 and 3.6+/-0.62 in low and high risk group of hydatidifrom mole patients, 3.5+/-0.70 and l.8 +/-0.52 in low and high risk group invasive mole patients and 1.5+/-0.91 and 0.3+/-0.15 in low and high risk group of choriocarcinoma patients respectively. As being prog- ressed the disease, the ratio was decreased(P<0.05) but there were no signifieant differences between high risk group af hydatidiform mole and low risk group of invasive rnole, high risk group of invasive mole and low risk group of choriocarcinoma. 2. Serum levels of PP10 were 26.4+/-12.50ng/ml and 3.5+/-1.31ng/ml in low and high risk group of hydatidigrom mole patients respectively. PP10 was not detected in invasibe mole and choriocarcinoma patients. 3.The response to chemotherapy was better in the group with higher SP1/ B-hCG ratio than in the group with lower SP1/B-hCG ratio. 4. Differential diagnosis between high risk group of hydatidifrom mole and low risk group of invasive mole could be made by measuring the SP1/ B-hCC ratio and serum levels of PP10. Frorn the above result, it is considered that the SP1/B-hCG ratio and serum level of PP10 can be helpful for predicting the prognosis of trophoblastic diseases, performing the appropriate chemotherapy and making differential diagnosia between high risk group of hydatidifrom mole and low risk group of invasive mole.