The serum expression of a novel tumour marker, CA 242, defined by monoclonal antibody C 242, was studied in 179 patients with pancreatic cancer. The results were compared with CA 19-9, CA 50 and CEA. CA 242 is a carbohydrate closely related, but not identical, to CA 19-9 and CA 50. The overall sensitivity of the CA 242 assay was 74%: 55% in stage I, 83% in stage II-III and 78% in stage IV disease. The specificity calculated from 112 patients with benign diseases was 91%. CA 19-9 had a higher sensitivity of 83%, but the specificity was only 81%. When comparing the markers by receiver operating characteristic analysis, the sensitivities were almost identical at all specificity levels. The CA 242 level was elevated in 7%, 15% and 7% of patients with benign pancreatic, biliary and liver disease respectively. The corresponding figures for CA 19-9 were 19%, 28% and 15% respectively. The sensitivity of CA 242 was higher than that of CA 50 and CEA at all specificity levels. In conclusion, tumour marker CA 242 seems to be a useful diagnostic tool for the diagnosis of pancreatic cancer, and is an alternative to CA 19-9. The advantage of CA 242 over CA 19-9 is its higher specificity when using the recommended cut-off levels of the assays.