Several characteristics of lymphomas of mucosa-associated lymphoid tissue (MALT type) suggest that they are antigen-dependent and that their growth parodies a normal immune response. We have previously shown that three cases of low-grade B-cell MALT-type lymphoma recognize autoantigens. In this study, we investigated the response of three low-grade and one high-grade case of MALT-type lymphoma to anti-idiotypic antibody as a model of antigen binding either alone or as a co-stimulus with B-cell mitogens. We also studied the response of tumour cells to interleukin-6 (IL-6), which induces differentiation to immunoglobulin-producing cells in many systems. Of the four cases studied, one low-grade case showed markedly enhanced proliferation in response to anti-idiotype alone. This could not be increased by the addition of mitogens. In the remaining two low-grade cases, mitogen responsiveness was observed which was affected by anti-idiotype either by an enhancement or by a reduction in the proliferative response. The high-grade case failed to respond to the stimuli studied. No response to IL-6 was observed. This study supports the suggestion that antigen may affect the pathogenesis of low-grade tumours of MALT type.