A new human cell line (OCUG-1) was established from peritoneal effusion of a patient with malignant gallbladder carcinoma. OCUG-1 cells proliferated mainly in clusters of cells partially floating in a monolayered sheet. The population doubling time is 47.1 h. DNA analysis showed that OCUG-1 cells were aneuploid and had two G(0)/G(1) peaks. The number of chromosomes was distributed in a broad range from 52 to 139. Subcutaneous injections of the cells induced tumor formation in all nude mice. The reconstituted tumors were poorly differentiated adenocarcinoma. High levels of SLX, CA19-9, SPan-1 and TA-4 were found in the serum of the original patient, but OCUG-1 cells produced only TA-4. We speculate that OCUG-1 may be a transitional form from adenocarcinoma to squamous cell carcinoma. Since OCUG-1 produces a high level of TA-4, it will be useful for the study of the biological nature of this carcinoma and the relationship between the expression of TA-4 and squamatization.