The expression of lactoside-binding lectin L-31 was analyzed in normal mucosa and in primary and metastatic gastric carcinomas. Immunoblotting revealed L-31 lectin in extracts of normal and malignant gastric tissues from 26 patients. The L-31 level was higher in tumor than in normal tissue in 9/26 cases, similar in 14/26 cases, and lower in 3/26 cases. Anti-L-31 monoclonal antibodies (MAbs) were used in immunohistochemical analyses to compare lectin expression in specimens of primary gastric carcinomas and adjacent normal mucosa from 39 patients and in specimens of metastases and the corresponding primary gastric carcinomas from 74 patients. The lectin was detected in normal gastric epithelial cells and in all gastric carcinoma specimens, albeit in varying amounts. The L-31 level was significantly higher in the primary tumor than in adjacent normal tissue in 55% of the well-differentiated tubular carcinoma cases and in 50% of stage-III and -IV tumors. L-31 expression in liver metastases from well-differentiated tubular primary gastric carcinomas was higher in 31% of the cases relative to the corresponding primary cancers. Likewise, L-31 expression in metastases from poorly differentiated gastric carcinomas in lymph nodes was higher in 38% of the cases compared to the primary cancers. The higher expression of L-31 in primary cancers and metastases of certain types implicates this lectin in the metastatic phenotype, but the presence of L-31 in a primary cancer is not sufficient to allow the metastatic propensity of the tumor to be predicted.