Passive Haemagglutination: A Sensitive ~-Foetoprotein Assay.Sum,mary. Passive haemagglutination for c¢x-foetoprotein was carried out in the reverse system coupling an¢i-ccz-foetoprotein-IgG with bis-diazotized benzidine on sheep erythrocytes. The lowest concentration detectable in the serum was 30 ng/ml. All the sera of 26 unselected patients with primary]iver cell carcinoma showed titers of 1:32 or higher. Titers of >1:4 were found in 142 patients with other carcinomas with or without liver metastases in 28.2% and in 238 patients with other benign liver diseases in 10.5% to 23.8%. A negative result in passive haemagglutination seems to elimin~te the diagnosis of a hepatocellular cancer; titers between 1:4 and 1:256 occur in hepatomas and in other carcinomas as in benign liver diseases; titers of 1:512 or higher are typical for primary liver cell carcinoma. %-foetoprotein is not a ~umor antigen specific for primary liver cell carcinoma or teratoblastoma. Nevertheless, regarding the enormous difference in the concentration in controls, in patients with benign liver diseases, malignant tumors and primary liver cell carcinoma, it is a higlfly characteristic tumor antigen.