Delayed bortezomib administration significantly increases serum cytokine levels in mice with GVHD. B6 (H2^b) recipients of BALB/c (H2^d) 15 million BMCs with or without 20 million spleen cells (SCs) were treated with or without 15 μg bortezomib per dose daily, day 12 to 13 after BMT. Serum was collected at 6 hours after bortezomib administration (day +13). Significant increases in TNF-α, IL-1β, and IL-6 were observed in delayed bortezomib-treated recipients with SCs compared with vehicle control-treated recipients with SCs (P < .05). Representative data from 1 of 3 independent experiments are presented, 5 serum samples per group.