A 180,000 MW molecule has been identified on porcine leucocytes that is the homologue of the 215,000/300,000 MW WC1 (T19) leucocyte antigen previously considered to be restricted to ruminants. In ruminants the WC1 molecule is expressed by a T-cell subpopulation that is CD2-CD4-CD8-CD5+ and that is gamma delta T-cell receptor positive (TcR+). In pigs, the 180,000 MW molecule, identified by a new monoclonal antibody CC101, is expressed by a gamma delta TcR+ T-cell subpopulation that is also CD2-CD4-CD8-. The p180+ cells are a major T-cell subpopulation comprising approximately 40% of the peripheral blood mononuclear cells from 6-9-month-old pigs. Expression of p180 identifies the majority of the CD2-CD4-CD8- T cells in porcine blood. The p180+ T cells have a distribution in lymphoid tissues that is distinct from that of T cells that express the CD2, CD4 or CD8 molecules. They are evident particularly in the thymic medulla, the epithelium, lamina propria and interfollicular areas of the small intestine, and the superficial dermis of the skin, but largely absent from conventional T-dependent areas of secondary lymphoid tissue.