HORIE, Y., CHIBA, M., IIZUKA, M., IGARASHI, K. and MASAMUNE, O. HLA Antigens on Colorectal Adenoma and Cancer Cells. Tohoku J. Exp. Med., 1990, 160 (4), 311-322-HLA-DR and -ABC antigens on adenoma and cancer cells of the colon and rectum were investigated. Fifteen specimens of adenomas from 15 patients with sporadic colorectal adenoma, seven specimens of adenomas from seven patients with familial polyposis coli (FPC), and 10 specimens of cancers from 10 patients with colorectal cancer were obtained. Normal colonic mucosa far from the lesions, taken from 15 patients with sporadic adenoma or cancer, served as normal control mucosa. HLA antigens were identified using an immunoperoxidase staining method. Epithelia of all normal control mucosa (n=15) expressed HLA-ABC antigens, but not HLA-DR antigens. HLA-DR antigens were expressed on 47% (7/15) of sporadic adenomas, 71% (5/7) of adenomas in FPC, and 100% (10/10) of cancers. The extent of HLA-DR expression on adenoma and cancer cells became broader with more severe dysplasia in adenoma, and increased undifferentiation in cancer. It also became broader with increasing mononuclear cell infiltration in both adenomas and cancers. HLA-DR antigens on adenoma and cancer cells appeared to be related to the neoplastic transformation of the epithelia, and to the mononuclear cell infiltration. Partial disappearance of HLA-ABC antigens on adenoma and cancer cells was observed in a few specimens of both adenomas and cancers.