A 72-year-old woman, who has been administered prednisolone and azathioprine with diagnoses of idiopathic thrombocytopenic purpura (ITP) and autoimmune hepatitis (AIH), underwent a complete medical examination because of monoclonal gammopathy (IgG-kappa). Tumors were found in the ileum and descending colon. Pathological examination of biopsy specimens suggested a diagnosis of marginal zone B-cell lymphoma of the MALT type with a high-grade component. Flow cytometric analysis by two-color staining revealed that the neoplastic B cells expressed CD38, CD19, IgG and kappa, but not CD5 or CD10. There were no abnormal plasma cells in bone marrow smears. The patient achieved complete remission after receiving three cycles of THP-COP chemotherapy, which resulted in a decrease of the IgG level to within the normal range. These findings indicated that monoclonal IgG-kappa might be produced by lymphoma cells. However, the relationship of the immunosuppressive agents to the pathogenesis of the MALT lymphoma remains to be clarified.