The unlabelled antibody-enzyme method with the peroxidase-antiperoxidase complex (PAP-complex) was used to demonstrate carcinoembryonic antigen (CEA) or CEA-like substances in paraffin-embedded specimens of tumour tissues with an anti-CEA-antiserum of a rabbit. The anti-CEA-antiserum used in this study exhibited a high anti-CEA-activity and a low activity to an impurity. The paraffin-embedded specimens of tumour tissues were investigated at serum dilutions of 1 : 16, 1 : 64, and 1 : 256. The sections of colorectal cancer showed the highest content of CEA. In 36 of 37 cases the tumour tissue reacted to the anti-CEA-antiserum up to a dilution of 1 : 256. The stomach cancer tissues contained significantly lower CEA-levels than the colorectal cancer. The sarcomas, malignant melanomas, the tumour of testis, the prostatic cancers and basal cell carcinomas of the skin were found to contain none or very low CEA-content. In most specimens of these tumours the CEA was not demonstrable at a 1 : 16 dilution of the primary antiserum.