This study was performed to distinguish between the coccoid form or spiral forms of Helicobacter pylori (Hp) and to elucidate the pathologic significance of these shapes of Hp. Specimens obtained from human or Mongolian gerbil stomachs were fixed in Carnoy's solution and embedded in paraffin. Sections 3 or 10 microm thick were stained with polyclonal anti-Hp antibody by the immunoperoxidase method. Stereo pairs were prepared from these thick sections by computer-assisted reconstruction. The two shapes of Hp were easily distinguishable by this method. In the human stomach, the proportions of Hp in the surface mucous gel layer (SMGL) and the gastric pits were 31.5% and 68.5%, and the percentage of the spiral form varied from 31.9% to 66.3%. In Mongolian gerbils, a higher proportion of Hp colonized the SMGL, and the spiral form existed more frequently both in the SMGL and on the surfaces of the surface mucous cells.