The distribution of hyaluronan (HA) in normal gastrointestinal wall and in tumors originating from their epithelium was studied using a specific probe prepared from cartilage proteoglycan (bHABC, biotinylated hyaluronan binding complex). The normal stratified squamous epithelium of esophagus showed an intense HA staining in the basal and lower intermediate layers, whereas the simple epithelia in the stomach and large intestine were HA negative. Esophageal in situ carcinomas expressed HA also in the cell layers close to the luminal surface, in regions normally negative. Most of the invasive squamous cell carcinomas maintained their HA expression, but in very poorly differentiated types the tumor parenchyma was devoid of HA. In both gastric and colonic adenocarcinomas the tumor parenchyma showed no HA. The stromal tissue was intensely HA positive in all tumors. Cancer cells invading the intestinal smooth muscle were surrounded by copious amounts of HA, whereas the muscular layer was otherwise very poor in HA staining. These results show that relatively well differentiated carcinoma cells themselves retain the high or low HA expression pattern of their original epithelium, whereas tumors stimulate HA deposition in the surrounding stroma.