Tubercidin is an adenosine analogue (7-deazaadenosine) which was highly cytotoxic to L5178Y murine leukemia cells in culture. Its effective range of cytocidal concentration was extremely narrow. Treatment of 0.3㎍/㎖ tubercidin for 2 hrs. led to 90-95% cell kill. Tubercidin at lower concentrations inhibited RNA synthesis slightly more than DNA synthesis in L5178Y cells. The effects of tubercidin on ³H-adenosine and ³H-uridine incorporation to nucleic acids were very similar, indicating that its action is not at the transport or adenosine kinase sites. Our results showed that ribosmal RNA and nucleolar RNA were most sensitive to tubercidin. However, they were not completely inhibited by the cytotoxic concentration. Synthesis of messenger RNA was less sensitive to the drug than ribosomal RNA. Transfer RNA and other small cytoplasmic RNA were not inhibited by even higher concentrations. The sensitivity of messenger RNA with or without the poly (A) segments to tubercidin was indifferent. The combination of tubercidin with another antineopleastic RNA inhibitor, actinomycin D, did not exhibited a synergistic effect on RNA synthesis in L5178Y cells.