A case of malignant lymphoma with a helper activity of neoplastic cells is reported. On admission, a significant number of plasma cells of polyclonal nature were seen in the peripheral blood, and polyclonal hypergammaglobulinemia was seen. The biopsied lymph node showed poorly differentiated lymphocytic lymphoma with marked proliferation of plasma cells. At the terminal stage, the patient became leukemic in contrast with the disappearance of plasma cells from the peripheral blood. Although the leukemic cells failed to form sheep erythrocyte rosettes, they were considered to be of T-cell origin morphologically. Cytochemically, they had a "dot"-like pattern of alpha-naphthyl acetate esterase and acid phosphatase activity. Ultrastructurally, they had highly convoluted nuclei, and cytoplasmic clustered dense bodies. They showed marked helper activity on pokeweed mitogen-induced B-cell differentiation in vitro. This case may provide a novel view concerning the cause of hypergammaglobulinemia induced by lymphoproliferative disorders.