Cell variants that differ in their tumorigenicity and immunogenicity have been isolated from a BDIX rat colon adenocarcinoma cell line, DHD/K12. One variant, PRO, and the clones derived from it, are poorly immunogenic and induce progressive and metastatic tumors; the other one, REG, and its clones, are highly immunogenic and induce regressive tumors. When looking for a membrane marker distinguishing between PRO and REG lines, we obtained monoclonal hybridomas by immunizing BALB/c mice with PROb or REGb cell clones. Hybridoma FI IC, producing an IgM monoclonal antibody (MAb) was able to distinguish between the cell variants on membrane immunofluorescence. All REGb cells strongly express FI IC membrane antigen. On PROb cells, FI IC antigen expression is weak, as demonstrated by cytofluorimetric analysis, and limited to a fraction of the cell population. The FI IC membrane antigen is highly specific for the DHD/K12 cell line and the variants derived from it, but is not expressed on cells dissociated from the DHD transplanted tumor, from which DHD/K12 was established, suggesting that FI IC antigen emerged during cell culture. Fluorescence absorption on synthetic oligosaccharides demonstrated that FI IC antibody cross-reacts with A type 3, A type 4 and A type 5 chain blood group tetrasaccharides.