The expression of human tumor-associated antigens CO17-1A, GA73-3, BR55-2, GICA 19-9, and CA50 and of carcinoembryonic antigen was immunohistochemically studied in the colonic mucosa of 70 Sprague-Dawley rats. Fifty were treated with 1,2-dimethylhydrazine (DMH) (with EDTA as a vehicle), ten were treated with EDTA only, and ten were untreated normal rats. The tumors were histogenetically divided as: (a) adenocarcinomas arising from villous adenomas; (b) adenocarcinomas arising from lymphoid-associated mucosa (LAM); and (c) adenocarcinomas arising in flat mucosa. Of 44 colonic adenocarcinomas, BR55-2 was expressed in 41 tumors, CO17-1A in 40 tumors, GA73-3 in 38 tumors, and GICA 19-9 in 38 tumors. CA50 and carcinoembryonic antigen were not expressed in the tumors. The highest antigenic expression (number of cells) was observed in adenocarcinomas arising in villous adenomas and the lowest in those arising in flat mucosa. Adenocarcinomas arising in LAM had an intermediate expression. The expression of these antigens had no correlation to the localization of the tumor and to the differentiation. The expression of these antigens was similar in the non-lymphoid-associated normal colonic mucosa of the untreated, EDTA-treated, and DMH-treated rats. In DMH-treated rats, LAM demonstrated increased expression (number of cells) and increased staining intensity of these tumor-associated antigens. In six of the 50 DMH-treated rats, only LAM expressed carcinoembryonic antigen. CA50 was not expressed in the normal colon of untreated, of EDTA-treated, and of DMH-treated rats, nor was it in DMH-induced tumors. None of the tumor-associated antigens (GICA 19-9 and CA50 and carcinoembryonic antigen) was detected in serum. It is concluded that this animal model would be of value in the preclinical evaluations of monoclonal antibodies for therapy in humans.