Immunohistochemical expressions of type 1 blood group antigens were studied for 95 cases of thyroid tumors, including 29 follicular adenomas, 23 follicular carcinomas, and 43 papillary carcinomas, applying monoclonal antibodies against DU-PAN-2, CA19-9, Lewis(a) (Le(a)), and Lewis(b) (Le(b)). Normal thyroid tissue invariably failed to express all four antigens. In follicular adenomas, DU-PAN-2 and CA19-9 were focally expressed in 7% and 21% of cases, and in follicular carcinomas, CA19-9 expression was limited to one case (4%); all cases were negative for DU-PAN-2. No or little expression of Le(a) or Le(b) was observed in these follicular tumors. In contrast, DU-PAN-2, CA19-9, Le(a), and Le(b) were expressed in 98%, 84%, 33%, and 49% of 43 papillary carcinomas, respectively. The positive stainings were observed mainly on the luminal surface of the tumor cells. The number of tumor cells that expressed DU-PAN-2 generally was greater than that of tumor cells that expressed CA19-9, Le(a), or Le(b). There was no significant difference in antigen expressions in female papillary carcinomas between subjects who were younger and older than 50 years old. The results suggest that DU-PAN-2 would be a useful immunohistochemical marker for distinguishing papillary carcinomas from follicular tumors. These immunohistochemical profiles imply the following: the activity of alpha2-3 sialyltransferase, a specific glycosyltransferase, would be more strongly enhanced in papillary carcinomas than in follicular tumors; the antigen expressions in papillary carcinomas may not be related to the alteration of the female sex hormone environment.