Human leukocyte interferon inhibits the proliferation of the virus-transformed human embryonic cells RSa. Incorporation of [3H]thymidine (TdR) into an intracellular pool and the activity of TdR kinase were reduced in the interferon-treated RSa cell culture. High-degree (90%) inhibition of [3H]TdR incorporation was associated with concentrations of added interferon that produced more than a twofold increase in the intracellular cyclic AMP (cAMP) level, and low-degree inhibition was associated with smaller increases in cAMP. In the IFr cell culture, which is relatively resistant to the anticellular action of interferon, considerably less inhibition of TdR incorporation and a slight increase in cAMP were observed. Extracellularly added dibutyryl-cAMP inhibited the proliferation of both RSa and IFr cells to almost the same degree. A decrease in cAMP level and the initiation of DNA synthesis of G1-phase-arrested RSa cells by serum addition were prevented when cells were pretreated with interferon. These results indicated that intracellular cAMP may mediate the inhibitory effect of interferon on DNA synthesis and cell growth.