The CA 50 levels in serum samples from 440 patients were estimated using a dissociated enhanced lanthanide immunofluorimetric assay. The distribution was similar to CA 50-RIA assays. Raised levels (greater than 14 U/ml) were present in 95% pancreatic cancer, 68% hepatoma, 54% advanced colorectal cancer, 58% advanced breast cancer and 48% lung cancer. High values were observed in adenocarcinoma of the lung, and were related to tumour mass in small cell lung cancer. CA 50 is independent of CEA. The marker is of considerable potential in pancreatic cancer where the majority of patients express the Can 50 Ag.