We have previously reported that Fas cross-linking resulted in an increase in phospholipase D activity in A20 murine cells (J.-S. Han et al., Arch. Biochem. Biochem. 367, 233-239, 1999). In an attempt to explore the Fas downstream factor contributing to the activation of phospholipase D, we have investigated the possible involvement of a small GTP hiding protein Res in signaling events that were triggered by Fas cross-linking, Upon adenoviral expression of dominant negative mutant of Res (N17Ras), an increase in phospholipase D activity by anti-Fas monoclonal antibody was diminished. Also, the Fas downstream signaling events triggered by Fas cross-linking such as the activation of phosphatidylcholine-specific phospholipase C, the increase in diacylglycerol level, and the translocation of protein kinase C to membrane fraction were all reduced by N17Ras expression. When parallel experiments were performed with manumycin-A, a Ras farnensyltransferase inhibitor, almost identical inhibitory effects on Fas downstream signaling were exhibited. These data suggest that Ras GTPase is essential in transmitting phospholipase D activation signal induced by Fas cross-linking and is located at phosphatidylcholine-specific phospholipase C upstream in Fas signaling cascades.