The expression and the role of Notch receptors and ligands during the induction of an immune response were investigated. I have shown that components of the Notch pathway were present in peripheral CD4 and CD8 T cells. Upon culturing and activation in vitro, in many Notch components were differentially expressed, in particular targets of Notch activation. In contrast, bone marrow-derived dendritic cells (DCs) expressed only low levels of Notch targets. However, the Notch ligand Jagged1 was strongly upregulated upon maturation of DCs with TNFα or LPS. By regulating the expression of the ligands, a role for antigen presenting cells (APCs) as the “signalling cells” is likely, whereas T cells expressing Notch receptors and Notch target genes may be the “receiving cells”. To investigate this hypothesis in vivo, transgenic mice with inducible overexpression of Jagged1 or Delta1 in DCs were generated. An in vitro approach to study the role of Notch ligands on APCs was carried out using an I-A^b transfected murine fibroblast cell line (I-A^b+ L cells) with endogenous B7.1 expression. I-A^b+ L cells co-transfected with Jagged1 or Delta (Jagged1⁺ or Delta1⁺L cells, respectively) were used as APCs in a mixed lymphocyte reaction (MLR) in vitro. Jagged1⁺ and I-A^b+ L cells induced similar levels of allogeneic T cell proliferation, whereas Delta1⁺ L cells had a slightly increased capacity to activate T cell proliferation. There were no phenotypical differences or changes in the level of apoptosis observed between T cells activated by Jagged1⁺, Delta1⁺ or I-A^b+ L cells. However, IFNγ secretion by T cells in response to Jagged1⁺ L cells was strongly reduced, whereas Delta1⁺ and I-A^b+ L cells induced normal levels of IFNγ secretion. There were no significant differences neither in the level of intracellular IFNγ nor of IFNγ transcripts between T cells in response to Jagged1⁺, Delta1⁺ or I-A^b+ L cells. Therefore I hypothesise that Jagged1-induced Notch signalling may be involved at the level of IFNγ secretion.