Molecular size of SPan-1 antigen and co-expression between SPan-1 epitope and Lewis phenotype determinants (LPD) on the same molecule in sera from patients with pancreatic, gastric and colon cancer and in culture medium from cancer cell line were studied. Column chromatography study on culture medium and sera showed that SPan-1 immunoreactivity was found in the void volume region as single major peak, or followed by one or two minor peak in the included volume. All of 4 pancreatic cancer cell lines showed molecular size heterogeneity. Co-expression between SPan-1 epitope and LPD on the same molecule was recognized in sera from cancer patients with Lewis a or Lewis b positive, but not in patients with Lewis a-, b-. Molecular carrying SPan-1 epitope had more frequently SPan-1 epitope than LPD in sera from cancer patients. These findings suggest that molecular size heterogeneity of SPan-1 antigen was more often in pancreatic cancer cells and overexpression of SPan-1 epitope on the same molecule seems to be specific for cancer.