Mammalian farnesyltransferase, which attaches a 15 carbon isoprenoid, farnesyl, to a cysteine in p21ras proteins, contains two subunits, alpha and beta. The beta subunit is known to bind p21ras proteins. We show here that the alpha subunit is shared with another prenyltransferase that attaches 20 carbon geranylgeranyl to Ras-related proteins. Farnesyltransferase and geranylgeranyltransferase have similar molecular weights on gel filtration, but are separated by ion exchange chromatography. Both enzymes are precipitated and immunoblotted by multiple antibodies directed against the alpha subunit of farnesyltransferase. The two transferases have different specificities for the protein acceptor; farnesyltransferase prefers methionine or serine at the COOH-terminus and geranylgeranyltransferase prefers leucine. The current data indicate that both prenyltransferases are heterodimers that share a common alpha subunit with different beta subunits.