Epstein-Barr-virus (EBV)-positive Burkitt's-lymphoma (BL) cell lines are not recognized by EBV-specific T cells, due to their non-immunogenic phenotype and restricted expression of latent EBV genes. We tested whether triggering of CD40 can alter the phenotype of the tumor cells with regard to: (i) expression of surface markers, (ii) expression of viral antigens, (iii) presentation of endogenous antigens to MHC-class-1 restricted cytotoxic T lymphocytes (CTLs), (iv) stimulatory capacity in allogeneic mixed-lymphocyte cultures (MLCs), (v) sensitivity to natural-killer (NK)-cell-mediated lysis. Co-culture of EBV-positive BL cells with CD40-ligand-transfected L cells induced up-regulation of CD54 and CD80 but did not affect the expression of viral genes. In spite of significant up-regulation of TAP1 and TAP2, and increased expression of MHC class 1, the BL cells remained unable to present endogenously expressed viral antigens to EBV-specific CTL. However, the up-regulation of adhesion and co-stimulatory molecules was associated with increased stimulatory capacity in MLC and enhanced sensitivity to NK cells. These findings indicate that, while inducing only a modest phenotype shift, cross-linking of CD40 under physiologic conditions may selectively enhance the sensitivity of BL cells to anti-tumor immune responses.