Panax ginseng roots have long been used as a medicinal herb in oriental countries. We have investigated anti-proliferative effects of lipid soluble Panax ginseng components on human renal cancer cell lines. Petroleum ether extract of Panax ginseng roots (GX-PE) or its partially purified preparation (7:3 GX) was added to cultures of three human renal cell carcinoma (RCC) cell lines, A498, Caki-1, and CURC II. Proliferation of RCC cells was estimated by a [3H]thymidine incorporation assay and cell cycle distribution was analyzed by flow cytometry. GX-PE, 7:3 GX, panaxydol and panaxynol inhibited proliferation of all three RCC cell lines in a dose dependent manner in vitro with an order of potency, 7:3 GX > panaxydol > panaxynol = GX-PE. Additive effect of interleukin 4 was also demonstrated, most prominently in Caki-1 which responded poorly to GX-PE alone. Analysis of cell cycle in CURC II and Caki-1 treated with GX-PE demonstrated increase in G1 phase population and corresponding decrease in S phase population. The present study demonstrated that proliferation of human RCC cell lines were inhibited by lipid soluble components of Panax ginseng roots by blocking cell cycle progression at G1 to S phase transition.