A monoclonal antibody, HML-1, was produced by fusion of NSI myeloma cells with spleen cells of a mouse immunized with isolated human intestinal intraepithelial lymphocytes (IEL). Immunofluorescence studies of isolated cells, as well as immunoperoxidase staining of tissue sections, indicated that HML-1 labeled all the various subsets of human intestinal IEL, approximately 40% of lamina propria T cells, 30% mesenteric lymphoblasts and some lymphocytes in other mucosae, particularly IEL. Conversely, it revealed only rare cells in all other lymphoid compartments. Analysis by polyacrylamide gel gradient electrophoresis showed that HML-1 precipitated two major noncovalently bound components of approximate mol. masses of 105 and 150 kDa from human IEL. HML-1 thus defines a novel human membrane antigen present on a subpopulation of lymphocytes preferentially associated with epithelia, and particularly with the intestinal epithelium. The characteristics of this human antigen are very similar to those of an antigen we had previously described in the rat. The possible functional role of this novel class of lymphocyte membrane antigens as well as the nature of the mechanism that triggers their expression remain to be elucidated.