The clinical significance of serum 2----6 sialyl Lewisa antigen was evaluated using "SLA 2----6 Otsuka" kits. Results indicated that the antigen was frequently elevated in sera obtained from patients with various cancers, including pancreas (73%), liver (67%), bilialy tract (66%), uterus (35%), and stomach (33%). Among other tumor markers examined, CA 19-9 (2----3 sialyl Lewisa) had a very similar cancer spectrum as 2----6 sialyl Lewisa. In the sera of patients with malignant disorders of digestive and respiratory organs, including stomach, intestine, pancreas, biliary tract and lung, the serum levels of CA19-9 tended to be higher than those of 2----6 sialyl Lewisa, usually exceeded those of CA19-9. This suggests that the 2----6 sialylation of Lewisa antigen is equally observed in malignant and non-malignant diseases, while the 2----3 sialylation is relatively specific to cancers. As a result, the ratio of the two antigens, CA 19-9: 2----6 sialyl Lewisa antigen ratio, exceeded 2.0 in most of the sera obtained from patients with malignancy, whereas the ratio was below 2.0 in most of patients with corresponding non-malignant diseases of those organs. The determination of the ratio may be clinically useful in the differential diagnosis of the malignant and non-malignant diseases in those organs.