The T cell intracellular antigen 1 (TIA-1) is a 17-kDa cytoplasmic granule associated protein also designated as GMP-17, for granule membrane protein of 17 kDa. The GMP-17/TIA-1 molecule is expressed in cells possessing cytolytic potential and could be involved in the signaling cascade of Fas (CD95)-mediated apoptosis. Within hematopoietic cell lines, the 2G9 monoclonal antibody (mAb) reacts with about 90% of CD16+, 50 – 60% of CD8+, and less than 10% of CD4+ normal peripheral blood lymphocytes. It reacts with almost all monocytes and granulocytes. This antibody also reacts with CD4+ activated T-cell clones, activated NK cell clones, and Con activated thymocytes, but not with B lymphocytes or B-cell lines.