The results of 119 radioimmunoscintigraphies (RIS) in 113 patients with the 99mTc-labeled monoclonal anti-CEA-antibody BW 431/26 (Behring) have been analysed. The aim of our study was the estimation of the method's sensitivity and specificity under different aspects to find out for which indications and questions the 99mTc-RIS is useful. Colorectal primary tumours in 19 patients were scintigraphically detected with a sensitivity of 83% and a specificity of 100%; 3 out of 7 other tumour sites were localised correctly. 55 patients were examined during the follow-up of colorectal cancer. There were 17 out of 22 true positive findings of local recurrences (sensitivity 77%, specificity 88%). Liver metastases were imaged as hot lesions with only 41% sensitivity and 86% specificity. The detection of extrahepatic tumour sites is difficult because of the persistently high blood-pool activity of the monoclonal antibody and, in the pelvic area, the unspecific bowel activity. Skeletal metastases were recognised in 7 out of 9 cases. In 14 patients with other non-colorectal carcinomas, RIS was successful in single cases. It is not helpful, however, when searching for tumours of unknown origin or for the screening of patients with elevated CEA levels without tumour history. The high technical, methodological and time effort required by RIS is justified in the follow-up of cancer patients when conventional diagnostic procedures are inconclusive or the status of morphological findings remains unclear. The use of RIS as an unspecific screening tool in tumour diagnosis must be rejected because of the not completely explored risks of the examination. Repeated applications of monoclonal antibodies require controls of the patients' HAMA titers before performing RIS.