Abstract Contact hypersensitivity (CHS) is a CD8 T cell mediated response to skin sensitization with hapten and subsequent challenge. Hapten-primed CD8 T cell infiltration into the skin challenge site during elicitation of CHS requires prior infiltration and activation of Gr-1+CXCR2+cells expressing both FasL and perforin. This study tested factors generating these myeloid cells in the bone marrow (bm) of hapten sensitized mice. The myeloid cells were enriched from the bm of DNFB sensitized mice 2 hrs after hapten challenge by depleting CD3+, B220+, NK1+, F4/80+, class II MHC+and Ly6G+cells with >90% of the remaining cells expressing Gr-1, CXCR2, CD11b, CD11c, Ly6C, CCR2, and CX3CR1, the phenotype of inflammatory monocytes (IM), plus FasL. The FasL+IM were at low numbers in the bm of naïve and hapten sensitized mice but increased to peak numbers 2–6 hrs. after hapten challenge of sensitized wild type (WT) mice. The FasL+IM were also generated in the bm of hapten sensitized and challenged Rag−/−mice and T-cell depleted WT mice, indicating that their generation can occur independently of T cells. The FasL+IM were not generated following hapten challenge of sensitized mice with targeted deletions in TLR4, IL-1β receptor or IFN-γ genes or in sensitized WT mice treated with anti-TNF-α mAb during hapten challenge. However, treatment with anti-GCSF antibody during hapten challenge increased their numbers in the bm and increased the magnitude of the CHS response elicited. Collectively, these results indicate that haptenreactive CD8 T cell infiltration into hapten challenged skin requires the TLR4-, TNF-α- and IFN-γ-dependentgeneration of bm-derived IM expressing FasL and perforin and their activation within the hapten challenged skin site.