Donor T cells and donor accessory cells in the lung are significant producers of TNF-α after allogeneic BMT. Lethally irradiated B6D2F1 mice received BMT from either syngeneic (B6D2F1, □) or allogeneic (B6, ▪) donors as described in “Materials and methods.” Animals were analyzed at week 5 for (A) lung histopathology and BAL fluid cellularity, (B) BAL fluid TNF-α levels, (C) numbers of TNF-α-secreting T cells, and (D) TNF-α production by pulmonary macrophages upon restimulation with LPS for 4 hours in vitro. Data are presented as mean ± SEM and are from 1 of 2 comparable experiments; n = 5 per group; ^*P < .05, black bar versus open bar.