10 ml of blood (2 ml for infected pediatric patients) is separated by density centrifugation on Ficoll-hypaque gradients. The mononuclear cell fraction is harvested, washed and cultured in the presence of cyclosporine so as to allow outgrowth of the donor's EBV-positive cells and establishment of an immortalized EBV-positive B cell line. The viral DNA within this line can then be amplified by PCR using primers from the BamHI-K region of the EBV-genome, which is known to be highly polymorphic among the various EBV strains. The different sized fragments of the amplified region can be distinguished by migration on an agarose gel (Alfieri et al, 1996). In order to verify our results for identity between patients and blood donors we will also employ the EBNA-typing technique which is performed by western blot analysis using a defined EBNA-positive serum (Alfieri et al, 1996).