A common early response of eukaryotic cells to stimuli which activate their proliferation is an increase in intracellular pH (ref. 1). In animal cells this is caused by the activation of an Na+/H+ exchange system; in fungi and plants an H+-pumping ATPase is involved. The critical question is whether this intracellular alkalinization is merely coincident with the activation of cell proliferation or whether it is a regulatory signal. To increase intracellular pH bypassing the usual physiological stimuli (growth factors, hormones etc.) alkaline media or ammonia have been used in the past. Both approaches suffer from long-term toxicity effects and cannot be used in tumorigenic assays with whole organisms. We introduce here a more specific approach which involves expressing the gene for the yeast plasma membrane H+-ATPase in fibroblasts. The resulting cells have an elevated intracellular pH and acquire tumorigenic properties, suggesting that the yeast ATPase gene behaves as an oncogene in mammalian cells. These experiments support a crucial role of intracellular pH in the growth control of animal cells.