Poly-N-acetyllactosaminyl structures carry a variety of physiologically and pathologically important carbohydrate antigens and are presumed to have essential roles in the process of cellular recognition, differentiation, malignant transformation and cancer metastasis. Monoclonal antibodies, lectins and endo-beta-galactosidase are useful histochemical tools for detecting and analyzing poly-N-acetyllactosamines in tissue sections. I (branched structure) and i (linear structure) antigens recognized by monoclonal antibodies have been shown to be differentiation antigens in mouse embryo and mouse and human teratocarcinoma cells as well as in human erythrocytes. They are also oncofoetal antigens and are expressed in carcinoma cells in several tissues and organs. Immobilized lectins specific to poly-N-acetyllactosamine structures have been successfully applied for fractioning glycoproteins with poly-N-acetyllactosamine, but histochemical use of these lectins has been restricted to some animal tissues. Among them, pokeweed mitogen agglutinin was used to detect branched poly-N-acetyllactosamine in normal and malignant human colon, demonstrating that it has a highly selective affinity for colorectal carcinomas. Griffonia simplicifolia agglutinin-II staining following endo-beta-galactosidase digestion procedure revealed the presence of poly-N-acetyllactosamine structures with or without blood group-specificities in several normal human tissues. By using this procedure, it was demonstrated that the blood group-related antigens oncofoetally expressed in thyroid carcinoma cells are carried by poly-N-acetyllactosamines containing a domain susceptible to the enzyme digestion. Staining with lectins specific to poly-N-acetyllactosamine in combination with endo-beta-galactosidase digestion demonstrated that poly-N-acetyllactosaminyl structures ubiquitously and consistently produced in thyroid papillary carcinomas are highly heterogeneous in their chain length and branching status and quite different from those produced in other thyroid neoplasms. Staining with monoclonal antibodies or lectins combined with endo-beta-galactosidase digestion procedures have been proven to be powerful tools for localizing and analyzing different types of poly-N-acetyllactosamine structures in normal and malignant tissues.