Carbohydrate residues in tissue can be localized by a very sensitive, specific, and simple technique, utilizing a biotin-labeled lectin followed by an avidin-biotin-peroxidase complex (ABC). Distribution of peanut receptors in benign and malignant tissues was found to be quite different. Normal colonic mucosa occasionally shows small dot staining in the supranuclear cytoplasm, while the tumor cells of colonic adenocarcinoma tend to show diffuse cytoplasmic or membranous staining. Normal breast ductal epithelium shows membranous staining at luminal borders, while medullary carcinoma of the breast shows distinct membranous and cytoplasmic staining. The use of this avidin-biotin-peroxidase complex technique may contribute to understanding the process of tumorigenesis as well as providing a sensitive method to determine early stages of malignant transformation.