The role of TNF in the expression of GVHD and GVHD-related immunodeficiency was studied in a well-established murine GVHD model of bone marrow transplantation across minor histocompatibility barriers (B10.BR → CBA/J) both in vitro and in vivo. Splenocytes from animals with GVHD profoundly inhibited the proliferation of normal spleen cells in response to a wide range of stimuli in an MHC-nonrestricted fashion. Neutralizing mAbs to TNF reversed the ability of splenocytes from animals with GVHD to suppress the proliferation of normal splenocytes stimulated by the mitogen concanavalin A. Addition of rTNF enhanced the degree of suppression. This reversal was similar to that previously reported for IFNγ and leucine methyl ester treatment of the GVHD populations. All three components are necessary for suppression to occur because addition of rTNF to cultures in which suppression had been reversed by anti-IFNγ or leucine methyl ester treatment did not reconstitute suppression. Neutralization of endogenous TNF production in vivo resulted in an amelioration of clinical GVHD, but neutralization of endogenous IFNγ resulted in a more severe course. However, in vivo neutralization of either TNF or IFNγ post-BMT resulted in a decreased ability of splenocytes from animals with GVHD to suppress mitogen responses but did not affect the generation of the suppressor cell population. These findings support multiple roles for TNF and IFNγ in the patho-physiology of GVHD, including terminal cellular differentiation and/or regulation of effector cell function.