This study was undertaken to isolate groups of eel (Anguilla japonica) intestinal cells undergoing active endocytosis in vivo and to localize these cells within the intestinal epithelium. A monoclonal antibody (8F1) of the immunoglobulin M class containing kappa light chains was raised against such endocytic cells. Ninetythree percent of the 8F1-positive cells absorbed fluorescent isothiocyanate-conjugated dextran from the intestinal lumen in vivo during a 30-min incubation, and they retained the ability to absorb dextran in vitro. The 8F1-positive cells constitute about 15% of the whole dissociated, macromolecule-absorbing cells. This suggests that other types of endocytic cells exists in the epithelium. The 8F1-positive cell can internalize fixed Vibrio anguillarum in vivo. Immunohistochemical observations demonstrated that 8F1-positive cells are very few in number and are located exclusively in the surface epithelium of the intestine. Thus these cells may well be specified phagocytic cells, different from those playing a basic nutritional role.