Jacalin is a lectin which reacts with D-galactose. We have tested jacalin on 75 samples of different formalin- and alcohol-fixed tissues. A consistent cytoplasmic stain of the histiocytes was observed in paraffin-embedded tissues in all cases studied of reactive sinus histiocytosis, macrophages in clear centres of follicular hyperplasia, in tuberculosis granulomas and in osteoclast-like giant cells in a breast carcinoma. We failed to find any clear binding of jacalin to the cells of eosinophilic granulomas, giant cell tumors of tendon sheath, pleomorphic malignant fibrous histiocytomas, Hodgkin's disease, melanomas, nevi or signet ring cell carcinomas of the breast and stomach. It seems that jacalin is a good marker for free histiocytes/macrophages, not for fixed histiocytes and tumors related to them. This lectin might play a role in differential diagnosis with histiocyte mimicking processes.