Concanavalin agglutinin (Con A) binding sites were studied in paraffin embedded lymph node specimens of reactive follicular hyperplasia (12 cases) and follicular lymphoma (37) using the avidin-biotin-peroxidase complex method, and the results were compared with those of Peanut agglutinin (PNA) and lysozyme stains. Very similar to the PNA stain, two categories of Con A receptor sites were observed: cytoplasmic and cell surface. In the reactive lymph nodes, the cells showing cytoplasmic receptor sites (CR+ cells) corresponded to macrophage-histiocytes and possibly dendritic reticulum cells in the H & E stained sections, while those showing cell surface receptor sites (SR+) corresponded to lymphoid cells. Unlike the PNA binding, however, the staining reaction of SR+ lymphoid cells was weak, and another staining pattern, a dot-like stain, was observed in some lymphocytes, both SR+ and SR-. In follicular lymphomas, CR+ histiocytes were distinctly displayed within the follicular centers in 25 of 37 cases, including 12 cases in which PNA stains on adjacent or nearby sections were negative for intrafollicular macrophage-histiocytes. Similarly, Con A stains were positive for the intrafollicular CR+ cells in four of the five cases in which lysozyme stains were negative. Many of these intrafollicular CR+ cells contained inclusion-like cytoplasmic globules and/or vacuoles, a hallmark of the large CR+ cells of germinal centers. These observations suggest that macrophage-histiocytes of presumably germinal center origin are retained in neoplastic follicular centers in varying degrees, and Con A might be a useful marker for macrophage-histiocytes in paraffin-embedded routine pathological specimens, in addition to the currently accepted markers, PNA and lysozyme.