AIMS Hep Par 1 has been described as a specific marker of hepatocellular differentiation and its immunohistochemical use has been suggested as a helpful tool for hepatocellular carcinoma (HCC) diagnosis. Most metastatic liver tumours come from the gastrointestinal tract and usually can be distinguished from HCC only through histology. We evaluated by immunohistochemistry the specificity of Hep Par 1, studying the presence of the epitope that reacts with Hep Par 1 in primary gastric and colorectal cancers. METHODS A series of 39 cases of primary gastric and 18 cases of colorectal carcinoma were selected. Twenty-six of the 39 gastric carcinomas were of the intestinal type, six of the diffuse type, three of the mixed type and five had hepatoid differentiation. Two of the 18 colorectal adenocarcinomas were well differentiated, 14 moderately differentiated, one poorly differentiated and one was of the mucinous type. Five-microm sections were stained by immunohistochemistry using Hep Par 1 as primary antibody. RESULTS Immunohistochemical staining was observed in 26 gastric carcinomas (69%) and nine large bowel carcinomas (50%). Fifteen of the 26 positive-stained gastric cancers were of intestinal type, four of diffuse type and two of mixed type cases. All of the five hepatoid type cases stained positively. Two of the nine positively stained colorectal cancers were well differentiated, six were moderately differentiated and one was a mucinous type adenocarcinoma. The staining pattern was cytoplasmic and granular as described in benign and malignant hepatocytes. The percentage of immunostained cells was graded as follows: 0 (no staining); 1 (>0-5%); 2 (> 5-50%); 3 (> 50%). Of the 26 positive gastric tumours, 13 showed a staining score of 1, eight scored 2, and five scored 3. Four of the nine positive intestinal carcinomas showed a staining score of 1, and five scored 2. CONCLUSIONS Our results show that Hep Par 1 is a highly sensitive marker of hepatocellular differentiation as demonstrated by the expression in gastric tumours with hepatoid histotype. However, the frequent reaction with neoplastic cells of gastric and bowel carcinomas shows a low grade of specificity of this antibody.