In cases of fulminant amoebic colitis we have determined the interactions between Entamoeba histolytica trophozoites and immune cells in order to better understand the pathophysiology of amoebic colitis. Eleven specimens of amoebic colitis and five specimens of colon without amoebic lesions were studied. Trophozoites and immune cells were located by topographic stains, histochemistry and immunohistochemistry. Trophozoites were seen in both damaged and undamaged areas of the colonic mucosa. Specimens of fulminant amoebic colitis showed: (a) an increase in IgA+, IgG+ B cells and neutrophils; (b) a reduction in IgM+ B cells, CD8+ T cells, macrophages, eosinophils and mast cells; and (c) no change in the number of NK and CD4+ T cells. The cellular infiltrate in amoebic colitis may represent the combined effects of amoebic monocyte locomotion inhibitory factor and switching of IgM+ B cells to IgG+ and IgA+ plasma cells, induced by amoebic antigens. Tissue damage in the absence of trophozoites may result from ischaemia or host immune responses.