2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent immunosuppressant and a prototypic ligand for the aryl hydrocarbon receptor (AhR). In a graft versus host (GvH) response, treatment with TCDD suppresses a cytotoxic T-lymphocyte (CTL) response by Day 10, concurrent with a T-regulatory (Treg) like phenotype in the donor CD4+ T-cells, observed on Day 2. The phenotype in CD4+ T-cells has been characterized as the following: CD25high, CD62Llow, CTLA-4+, and IL-10+. TCDD-mediated suppression may be due to the induction of this T-regulatory cell, which leads to altered cytokine production. In addition to IL-10, IFN-g also plays an important role in T cell differentiation. Previous experiments have measured the effects of TCDD on cytokine production only on day 2 of a GvH response. We were therefore interested in seeing if changes in cytokine production persist through day 3, when the Treg phenotype is still evident. We tested supernatants from prior GvH experiments to determine levels of these cytokines using enzyme-linked immunosorbent assays (ELISA). The results of these assays indicate no significant difference in IL-10 cytokine production between day 2 and day 3. However, on day 3, IFN-g was increased in the supernatants of cells from TCDD-treated animals, indicating that TCDD up-regulated IFN-g production. There are other cytokines also produced by Tregs, for example IL-2, IL-27 and TGF-b, which were not quantified and which could be involved in TCDD-mediated immunosuppression seen in a GvH model.