Southern blotting analysis of genomic DNA extracted from NK-YS cells and fresh leukemic cells after restriction enzyme digestion using EcoRI and PstI with the EBV terminal repeat. EBV-transformed B lymphoblastoid cell line, LAD cells, and Jurkat cells were used for positive and negative controls, respectively. The NK-YS cells showed a monoclonal band of EBV terminal repeat identical to that of the fresh leukemic cells, but not identical to that of the LAD cells.