A case of undifferentiated malignant tumor of the stomach is reported. The immunohistochemistry of biopsy specimens pointed to a diagnosis of carcinoma, the tumor cells being cytokeratin positive and leukocyte common antigen (LCA) negative. After resection, however, histopathologic results showed that the tumor was a large cell lymphoma with plasmablastic differentiation. A new immunohistologic study confirmed, on the one hand, the diagnosis of lymphoma with its monotypic character IgA kappa and, on the other, positivity with three different cytokeratins of the lymphoma cells and their negativity with LCA. The aberrant immunophenotyping of this lymphoma is exceptional and must not undermine the recognized usefulness of LCA and cytokeratin, which both are basic antibody markers of immunohistochemistry in undifferentiated malignant neoplasms.