To test the Kirsten‐Ras (Ki‐Ras) alternative prenylation hypothesis in malignant transformation, we used a novel farnesyltransferase inhibitor competitive to farnesyl‐pyrophosphate, RPR130401, and a CaaX peptidomimetic geranylgeranyltransferase‐1 inhibitor GGTI‐298. In Ki‐Ras‐overexpressing transformed adrenocortical cells, RPR130401 at 1–10 μM inhibited very efficiently the [3H]farnesyl but not [3H]geranylgeranyl transfer to Ras. However, proliferation of these cells was only slightly sensitive to RPR130401 (IC50=30 μM). GGTI‐298 inhibited the growth of these cells with an IC50 of 11 μM but cell lysis was observed at 15 μM. The combination of 10 μM RPR130401 and 10 μM GGTI‐298 inhibited efficiently (80%) cell proliferation. These combined inhibitors but not each inhibitor alone blocked the cell cycle in G0/G1 and disrupted MAP kinase activation. Thus, combination of two inhibitors, at non‐cytotoxic concentrations, acting on the farnesyl‐pyrophosphate binding site of the farnesyltransferase and the CaaX binding site of the geranylgeranyltransferase‐1 respectively is an efficient strategy for disrupting Ki‐Ras tumorigenic cell proliferation.