Abstract: Disclosed are methods and compositions for the identification, characterization and inhibition of mammalian protein farnesyltransferases, enzymes involved in the farnesylation of various cellular proteins, including cancer related ras proteins such as p21.sup.ras. One protein farnesyltransferase which is disclosed herein exhibits a molecular weight of between about 70,000 and about 100,000 upon gel exclusion chromatography. Also disclosed are methods and compositions for the preparation of farnesyltransferase by recombinant means, following the molecular cloning and co-expression of its two subunits, for assay and purification of the enzyme, as well as procedures for using the purified enzyme in screening protocols for the identification of possible anticancer agents which inhibit the enzyme and thereby prevent expression of proteins such as p21.sup.ras.