Using a newly established HTLV-1 positive T cell line as an immunogen, a new monoclonal antibody, Ber-ACT8, was produced. It reacts with in vitro activated T cells and a small subset of normal resting T cells, but not with resting B cells or any of the 29 established human permanent cell lines tested. Immunohistological analysis of a wide spectrum of human tissues showed that Ber-ACT8 reactivity is restricted to a few T cells in the peripheral blood, the extrafollicular areas of lymph nodes and tonsils, and splenic red pulp. In the gut Ber-ACT8 labelled most intraepithelial T cells and up to 50% of lamina propria T cells. The antibody also immunostained T cells present in the oral and bronchial mucosa. Double labelling on splenic cells, fresh blood lymphocytes, and in vitro activated T cells showed that most Ber-ACT8 positive cells coexpressed CD8. Ber-ACT8 did not react with any of the 14 Hodgkin's lymphomas nor any of the 172 non-Hodgkin's lymphomas tested, with the exception of 10 cases of T cell lymphomas, five of which were located in the jejunum and associated with coeliac disease, and one B cell lymphoma, and most cases of hairy cell leukaemia tested. Parallel immunostainings with Ber-ACT8, anti-TCR-beta (beta F1), and anti-TCR-delta showed that most Ber-ACT8 positive T cells carry the TCR of alpha beta type. Comparison of Ber-ACT8 with HML-1, B-ly7, and LF61 showed essentially the same reactivity and an identical molecular target. The molecular structure recognised seems to be a trimeric molecule with components of 150, 125 and 105 kilodaltons, with the Ber-ACT8 epitope localised on the 150 kilodalton chain. The 150 kilodalton molecule contains an 0-linked carbohydrate moiety of about 10 kilodaltons. Because of its very selective distribution, the trimeric antigen is a powerful reagent for the diagnosis of gut T cell-derived T cell lymphomas and other extranodal T cell lymphomas, as well as hairy cell leukaemia.