The careful histologic examination of the macroscopically normal colon in rats treated 8 months previously with a single dose of the colonotropic carcinogen dimethylhydrazine (DMH) demonstrated atypical colonic glands in normally occurring lymphoid mucosal patches. The atypical glands were characterized by the absence of cytoplasmic mucus, hyperchromatic cell nuclei, cell stratification, and increased number of mitotic figures. In 61.7% of the experimental animals, atypical colonic glands were found beyond the boundary of the muscularis mucosae, but in none of the controls. Thus, microinvasive adenocarcinoma of the colon seems to be a common finding in rats treated with a single dose of DMH. These adenocarcinomas remain, however, undetected at macroscopic examination. Measurements performed in consecutive lymphoid aggregates demonstrated that 50% of rats receiving DMH had thicker lymphoid aggregates than control animals. In spite of this, no tumor was recorded at naked eye examination in these areas. The macroscopic examination appears to be an unreliable method of ruling out malignancy in experimental rats treated with a single dose of the above-mentioned carcinogen.