In addition to being carcinogenic, polycyclic aromatic hydrocarbons (PAHs) are known to cause deleterious effects on the immune system, including a marked reduction in bone marrow granulocytes and B lymphocytes. The molecular mechanisms underlying bone marrow hypocellularity are incompletely understood. Hematopoiesis is governed by the production of cytokines and the resultant signaling pathways that they initiate. Our hypothesis was that PAHs may disrupt cytokine production in the bone marrow resulting in the perturbation in bone marrow cellularity observed after PAH administration. TNF-alpha and IFN-gamma are two cytokines that are involved in the regulation of hematopoiesis. Based on observations made in previous research, we sought to determine if the effects of 7-12 dimethylbenz[a]anthracene (DMBA) on the murine bone marrow were mediated through the actions of these molecules. Transgenic mice that were null for either IFN-gamma or TNF-alpha receptors were injected with DMBA and the resulting bone marrow cellularity compared with wild-type mice. We observed that tumor necrosis factor alpha receptor (TNFR) null mice were protected against DMBA-induced bone marrow hypocellularity, while IFN-gamma null mice were not. In addition, we found that dsRNA-dependent protein kinase (PKR) null mice were also protected from DMBA-induced hypocellularity. PKR is an intracellular signaling molecule that has been demonstrated to be activated by TNFR-mediated signaling. Furthermore, we observed upregulation of PKR in the bone marrow after DMBA administration that was dependent on signaling through TNFR. These results point to a role for TNFR-dependent signaling, operating at least in part via PKR activation, as a mechanism for DMBA-induced bone marrow toxicity.