Abstract: Amaranthin, a new lectin isolated from the seeds of Amaranthus caudatus, is purified and characterized. Amaranthin, designated ACA (Amaranthus caudatus agglutinin), has a high affinity for the T-antigen glycoconjugate and variants thereof expressed by proliferating colorectal epithelium. ACA selectively binds glycoconjugates in the lower half of the colonic crypt and is useful as a marker of colonic epithelial cell differentiation in the human colon. ACA also binds to glycoconjugates in neoplastic colonic tissues, and binds to preneoplastic polypoid and flat colonic tissue from patients with familial polyposis coli. ACA is also readily adapted for quantitative measurement of lectin-binding sites. ACA binding assays can thus be used for detecting early stage abnormalities of proliferation and differentiation in normal-appearing preneoplastic colorectal epithelium and to diagnose neoplastic premalignant and malignant colorectal lesions.