A 67 year old woman with rheumatoid arthritis was admitted to hospital in acute renal failure. Her clinical features included increasing dyspnoea and oedema, and a computed tomogram of the abdomen showed a large mass in the retroperitoneum. Twenty six days later, she died, and a post mortem examination was carried out. The histological changes of the mass indicated B cell lymphoma of diffuse large cell type, with a reactive proliferation of erythrophagocytosing histiocytes. Immunocytochemical studies showed that the histiocytes were positive for CD-68 and lysozyme, but negative for S-100 protein. Such neoplastic B cell proliferation accompanied by activation of benign looking histiocytes with erythrophagocytosis is very rare.