The clone 4H9 recognizes the 40 kDa CD7 antigen abundantly expressed on the human T and NK lymphocytes. The CD7 antigen is expressed throughout T lymphocyte differentiation and is present on 85% to 90% of peripheral blood T cells. In normal individuals, the CD7 antibody reacts with all CD8+ T cells, approximately 90% of CD4+ T cells, and most NK cells. CD7 is weakly reactive with monocytes and does not react with granulocytes or B lymphocytes. It is also expressed on 50% of thymocytes. In leukemias, the CD7 antigen is present on most T lymphoid lineages.