Epstein-Barr virus (EBV) from P3HR-1 cells, but not from B95-8 cells, can induce the synthesis of thymidine kinase (TK) in TK-negative Raji cells. The synthesis of TK was slightly reduced, but not inhibited, when cells were cultivated in the presence of cytosine arabinoside (ara-C). On the other hand, the synthesis of TK in ordinary Raji cells was enhanced in the presence of the drug. Thymidine-beta-D-arabinofuranoside (ara-T) was capable of reducing the conversion rate of thymidine to TdR nucleotides by extracts prepared from superinfected Raji TK-cells, but had no influence on TK activity in cell extracts from ordinary Raji cells and EBV-negative Ramos cells. This suggested a broader substrate specificity of the virally induced enzyme.