The peroral small intestinal biopsies of 5 patients with chronic malabsorption leading to a fatal course were examined retrospectively for the presence of abnormal (? premalignant) cells. Techniques employed for their identification included routine histology, immunofluorescence, immunohistochemistry (horseradish peroxidase) and electron microscopy. The biopsies of 3 other patients, 2 with known lymphoma and 1 with alpha chain disease were examined for any similarity in the ultrastructural appearances of abnormal cells and their immune cellular behaviour in these 2 groups of patients. Fine structural identification of cells displaying either nuclear or cytoplasmic abnormalities was possible in all 8 patients and varied from easily identifiable cell types, like the plasma cell, lymphocyte or histiocyte to those possessing more than one characteristic feature. The immunological staining techniques did not help in the identification of these abnormal cells. We conclude that careful E.M. examination of the mucosal cellular infiltrate in patients with chronic malabsorption can make a valuable contribution in identifying diffusely abnormal cells, though it may not be possible to determine whether they are of histiocytic or lymphocytic origin.