The lectin-binding properties of human follicular and papillary carcinoma were studied histochemically and compared with lectin binding to normal or goitrous thyroid tissue. Well-differentiated minimally invasive follicular carcinoma showed a lectin-binding pattern essentially identical to those of the normal thyroid gland and benign adenomatous lesions. Overtly invasive follicular carcinoma showed focal reactivity with some lectins that were nonreactive with normal follicular thyroid cells (Solanum tuberosum and soybean in three of three cases; Ulex europaeus in two of three cases; and Dolichos biflorus, Laburnum alpinum, and peanut in one of three cases). In papillary carcinomas, the cells lining the papillary structures reacted focally with some lectins that did not bind to normal thyroid cells (S tuberosum and U europeaus in seven of seven cases; Helix pomatia, Helix aspersa, and soybean in four of seven cases; and peanut, Griffonia simplicifolia, D biflorus, and Vicia villosa in one of seven cases). All these lectins, as well as those reacting with normal thyroid cells, reacted more strongly with cells of papillary structures than with those forming solid nests and follicles. Despite these lectin-defined differences in the composition of glycoconjugates of benign and malignant thyroid cells, the inconsistent and focal nature of the changes precludes the use of lectins in diagnostic histopathology.