Coenzyme Q added to culture media stimulates the growth of HeLa and Balb/3T3 cells in serum free conditions. The stimulation by coenzyme Q is additive to the stimulation by ferricyanide, an impermeable electron acceptor for the transplasma membrane electron transport. alpha Tocopherylquinone can also stimulate cell growth, but vitamin K1 is inactive or inhibitory. The response to coenzyme Q and ferricyanide is enhanced with insulin. A contribution to plasma membrane NADH oxidation or modification of the membrane quinone redox balance can be a basis for the growth stimulation.