Farnesyl-Protein transferase catalyses a post-translational modification of Ras that is obligatory for the cell transforming activity of this oncogene protein. The screening of natural products to identify inhibitors of this enzyme as a potential anticancer agents, has led to the isolation of two novel dicarboxylic acids, named chaetomellic acids from Chaetomella acutiseta, as potent and selective inhibitors which appear to be the first examples of nonphosphorous containing FPP mimics.