Six patients who had hydatidiform mole and who subsequently developed clinical choriocarcinomas were studied. Serial plasma samples were assayed for HCG content by (i) haemagglutination-inhibition test (HI test), (ii) a non-specitic radioimmunoassay and (iii) a specific radioimmunoassay developed against the beta-sub unit of HCG. When levels of HCG were high all three assays gave results which parallelled each other. However, the radioimmunoassays were able to detect the presence of HCG several weeks after the HI test became negative. Luteinizing hormone cross-reacted in the non-specific radioimmunoassay of HCG and may lead to false positive results. The present study indicated that it is important to employ the radioimmunoassay against the beta-sub unit of HCG to permit a specific assessment of HCG activity and thereby provide a reliable monitoring of patients following hydatidiform mole or with choriocarcinoma.