Résumé : A prospective study of HCG-β (the human chorionic gonadotrophin and its free β subunit) and of H-α (the free α subunit) was carried out in the plasma of 414 patients with malignant tumours and 99 patients with benign diseases (chiefly benign tumours and benign conditions associated with tissue hyperplasia). HCG-β and excessive free α subunit concentrations were found with a similar frequency (in 16-21% of the patients); both were found as frequently in the benign and in the malignant conditions; furthermore there was no clearcut predisposition according to the pathology except for choriocarcinomas. However the benign and the malignant conditions differed from each other in that the concentrations of HCG-β were significantly higher in the latter whereas the very high plasma concentrations (> 50 ng/ml) of HCG-β or of H-α were specific for malignant conditions. The patients with HCG-β were not the same as those with excessive H-α concentrations: 75% of the latter had no detectable HCG-β, indicating that the production of the α subunit can be regulated independently from that of HCG-β. Unlike in the patients with excessive H-α, in the patients with HCG-β there was a very significant correlation between the HCG-β and the corresponding free α subunit concentrations: this indicates that the production of HCG-β was linked to that of the free α subunit whereas the reverse was not necessarily true. The concentrations of HCG-β in plasma were qualified according to the elution profiles on Sephadex chromatography and the relative affinity of our antibody to HCG and the free β subunit. The presence of the free β subunit and of abnormal chorionic gonadotrophin was demonstrated. © 1978.