We have previously described a monoclonal antibody (MoAb), H2, which recognized a tumour-unique antigen on a human T-cell chronic lymphatic leukaemia (T-CLL, CD3,4+). However, further characterization of H2 has revealed a reactivity with the majority of T lymphocytes and a minority of B lymphocytes, some malignant T cells and a few cell lines of leukaemia or of hematopoietic tumour origin. The molecular weight of the antigen (80,000) precipitated by the MoAb H2 from the cell lines NALM-6 and Reh corresponded to that previously found. When PBL were stimulated with PHA, IL-2, or Con A a reduced reactivity of H2 could be seen. The MoAb H2 was submitted to the Fourth International Conference on Human Leucocyte Differentiation Antigens, Vienna, 1989. H2 did not cluster in any of the 78 clusters of differentiation (CD 1-78) discussed at the conference, indicating its unique reactivity. This suggests that we have defined a new antigen on lymphocytes with a possible role along the resting-proliferating axis.