The small guanine nucleotide binding protein Ras participates in a growth promoting signal transduction pathway. The mechanism by which interaction of Ras with the protein kinase Raf leads to activation of Raf was studied. Raf was targeted to the plasma membrane by addition of the COOH-terminal localization signals of K-ras. This modified form of Raf (RafCAAX) was activated to the same extent as Raf coexpressed with oncogenic mutant Ras. Plasma membrane localization rather than farnesylation or the presence of the additional COOH-terminal sequence accounted for the activation of RafCAAX. The acti vation of RafCAAX was completely independent of Ras; it was neither potentiated by oncogenic mutant Ras nor abrogated by dominant negative Ras. Raf, once recruited to the plasma membrane, was not anchored there by Ras; most activated Raf in cells was associated with plasma membrane cytoskeletal elements, not the lipid bilayer. Thus, Ras functions in the activation of Raf by recruiting Raf to the plasma membrane where a separate, Ras-independent, activation of Raf occurs.