Elevated levels of inflammatory cytokines were detected after T-BsAb treatment in huCD3/CD20 double transgenic mice with a high tumor burden. Human CD3/CD20 double transgenic mice were engrafted with huCD20-expressing E2A-PBX1 leukemia cells. T-BsAbs (CD3 × CD20) were administered 8 days after leukemia engraftment to allow for growth into a high tumor burden. Serum was obtained at 3 hours and 24 hours after T-BsAb treatment, and cytokine levels were analyzed by the BD Cytometric Bead Array assay, using the Mouse Inflammation Kit.