The concentration of human chorionic gonadotrophin in fluid content from differently-sized groups of hydatid vesicles, serum and urine was determined in five patients with hydatidiform mole by an immunological method. The HCG concentration of fluid content from small hydatid vesicles (0·3−0·7 cm. in diameter) was significantly higher than that of fluid content from large hydatid vesicles (0·8−1·2 cm.). The HCG value of serum was always much lower than that of fluid content from both small and large hydatid vesicles. The vesicle/serum ratio of HCG concentration ranged from about 1·5 to 4·5. The urinary level of the hormone was higher than that of both serum and vesicular fluid, except in one case. The results have been related to the different hypotheses of the pathogenesis of hydatidiform mole. The results seem to support Hertig's hypothesis which considered (1) the trophoblastic hyperplasia to be a result of the enlargement of the villus-vesicle and (2) the hydatid fluid to be a product of the active trophoblastic tissue of the mole.