Farnesyltransferase catalyses the post-translational modification of proteins by a cholesterol precursor, farnesylpyrophosphate. One of the substrates of this enzyme is the product of the ras oncogene. Recently, inhibitors of farnesyltransferase have been identified through two different approaches: microbial screens for natural compounds, and substrate analogues. These inhibitors may be useful in blocking the action of Ras proteins, in further characterizing protein prenyltransferases, and in elucidating the regulation of cholesterol metabolism.