After isoprenylation, Ras and other CAAX proteins undergo endoproteolytic processing by Rce1 and methylation of the isoprenylcysteine by Icmt (isoprenylcysteine carboxyl methyltransferase). We reported previously that Rce1-deficient mice died during late gestation or soon after birth. We hypothesized that Icmt deficiency might cause a milder phenotype, in part because of reports suggesting the existence of more than one activity for methylating isoprenylated proteins. To address this hypothesis and also to address the issue of other methyltransferase activities, we generated Icmt-deficient mice. Contrary to our expectation, Icmt deficiency caused a more severe phenotype than Rce1 deficiency, with virtually all of the knockout embryos (Icmt-/-) dying by mid-gestation. An analysis of chimeric mice produced from Icmt-/- embryonic stem cells showed that the Icmt-/- cells retained the capacity to contribute to some tissues (e.g. skeletal muscle) but not to others (e.g. brain). Lysates from Icmt-/- embryos lacked the ability to methylate either recombinant K-Ras or small molecule substrates (e.g. N-acetyl-S-geranylgeranyl-l-cysteine). In addition, Icmt-/- cells lacked the ability to methylate Rab proteins. Thus, Icmt appears to be the only enzyme participating in the carboxyl methylation of isoprenylated proteins.