The aim of this study was to investigate the influence that endogenous IFNs released in response to antigenic or viral stimuli has on recognition of alloantigens in MLC. Results indicated that both the magnitude and the kinetics of response can be modified by IFNs. Neutralizing antibodies with specificity for IFN-gamma inhibit early and enhance late proliferative responses in MLC. Addition of physiological concentrations of IFN-gamma enhanced both early and peak proliferation, whereas IFN-alpha markedly inhibited alloantigen-induced lymphocyte proliferation. Further experiments revealed that IFN effects in MLC are not caused by direct interaction with responder cells: pretreatment with IFNs neither failed to alter their subsequent proliferative reactivity, nor did it influence production of IL 2 in MLC. IFN-gamma mainly affected MLC responses by direct interaction with stimulator cells. These influences on hemopoietic and non-hemopoietic stimulator cells were complex and could not simply be explained on the basis of an altered expression of class II MHC antigens. When induced by IFN-gamma to maximally express class II antigens, pbmnc, LCL or homogeneous populations of macrophages showed a marked deficiency to induce primary or secondary proliferative T cell responses. Resting unsensitized or sensitized T cells were not stimulated by class II MHC antigens constitutively expressed or induced by IFN-gamma on cell types other than dendritic cells or LCL. Class II antigens on the former cells were, however, readily recognized by T helper blasts, and this process involved the T4 epitope of the T cell receptor. IFN-gamma treatment also influenced the intrinsic suppressive capacity of macrophages or keratinocytes without involving prostaglandin synthesis or inducing expression of IL 2 receptors on non T cells.