2 cases of precursor T cell lymphoma and 37 cases of peripheral T cell lymphoma were investigated for their reactivity with the monoclonal antibody (mAb) HML-1, which recognises human intestinal T lymphocytes but not lymph-node T cells. In all but one of the lymphomas studied, the tumour cells were unreactive with the mAb HML-1. The HML-1+ lymphoma was the only tumour that was primarily localised in the epithelium and lamina propria of the small intestine, and was associated with ulcerative jejunitis and coeliac disease. This result suggests that the HML-1+ lymphoma was derived from intestinal mucosa T lymphocytes and differs from precursor T cell lymphoblastic lymphomas and nodal and cutaneous peripheral T cell lymphomas.