The CD137, expressed on activated T cells, is a reliable surrogate marker for antigen specific T cells. In-vitro, the authors examined the possibility for toxin-conjugated anti-CD137 mAb to kill CD137-expressing T cells. Doxorubicin and fluorescence were conjugated to anti-CD137 mAb. The authors observed that fluorescence-conjugated anti-CD137 was internalized into activated CD4+ T cells and CD8+ T cells within 4 hours after its treatment and these cell lines constitutively expressed CD137. Doxorubicin-conjugated anti-CD137 mAb induced apoptosis in a majority of CD4+ T cells and CD8+ T cells within 48 hours after treatment compared with control antibodies. The article further confirm the specificity of toxin-conjugated anti-CD137 mAb showing that only T cells that expressed CD137 underwent apoptosis by staining with toxin-FITC-mAb conjugates. The article also found that toxinconjugated anti-CD137 mAb was able to delete allore-active T cells in vitro. The result indicates that CD137 targeted delivery of toxin is a good strategy to delete antigen-specific T cells.