Microsomal and soluble peptidases from bovine liver and pig brain hydrolyze the farnesylated, Ras-based CAAX peptide [3H]Ac-fCVIM-OH. However, they differ in their sensitivity to substrate-based inhibitors, sulfhydryl and chelating agents, pH and ionic strength optima, and stability. The microsomal activity was exquisitely sensitive to the substrate-based inhibitor Boc-fC[CH2]VIM-OH, moderately sensitive to the sulfhydryl agent pCMB, but insensitive to NEM and the metal-chelating agent o-phenanthroline. The soluble activity was insentive to Boc-fC[CH2]VIM-OH, but very sensitive to pCMB, NEM and o-phenanthroline, suggesting it to be the previously reported (Biochem. Biophys. Res. Commun. 198, 787-794 (1994)) zinc metallopeptidase. The microsomal activity is most likely to be a cysteine peptidase involved in the post-translational processing of Ras proteins.