The farnesyltransferase (FTase) inhibitor FTI-277 is highly effective at blocking oncogenic H-Ras but not K-Ras4B processing and signaling. While inhibition of processing and signaling of oncogenic K-Ras4B is more sensitive to the geranylgeranyltransferase I (GGTase I) inhibitor GGTI-286 than it is to FTI-277 in K-Ras4B-transformed NIH3T3 cells, the sensitivity of K-Ras as well as H- and N-Ras to the CAAX peptidomimetics in human tumor cell lines is not known. Here, we report that a panel of five human carcinoma cell lines from pancreatic, pulmonary, and bladder origins all express H-, N-, and K-Ras, and their respective prenylation sensitivities to the FTase and GGTase I inhibitors is variable. In all of the cell lines investigated, the prenylation of N-Ras was highly sensitive to FTI-277, and in two of the cell lines, N-Ras showed slight sensitivity to GGTI-298, an analog of GGTI-286. Although the prenylation of H-Ras was also sensitive to FTI-277, complete inhibition of H-Ras processing even at high concentrations of FTI-277 and/or GGTI-298 was never achieved. The prenylation of K-Ras, on the other hand, was highly resistant to FTI-277 and GGTI-298. Most significantly, treatment of human tumor cell lines with both inhibitors was required for inhibition of K-Ras prenylation. In one cell line, the human lung adenocarcinoma A-549, prenylation of K-Ras was highly resistant even when co-treated with both inhibitors. Furthermore, soft agar experiments demonstrated that in all the human tumor cell lines tested inhibition of K-Ras prenylation was not necessary for inhibition of anchorage-independent growth. In addition, although GGTI-298 had very little effect on soft agar growth, the combination of FTI-277 and GGTI-298 resulted in significant growth inhibition. Therefore, the results demonstrate that while FTI-277 inhibits N-Ras and H-Ras processing in the human tumor cell lines evaluated, inhibition of K-Ras processing requires both an FTase inhibitor as well as a GGTase I inhibitor, and that inhibition of human tumor growth in soft agar does not require inhibition of oncogenic K-Ras processing.