THE pioneering studies of Hayflick¹ showed that normal human diploid cells of the WI-38 strain have a limited lifespan of approximately 50±10 population doublings (PDL) in vitro. It is still not known to what extent this phenomenon is the result of the artificiality of the cell culture system : factors such as nutrition and ambient environmental factors may restrict the ability of these cells to express their true genetic potential. To what extent genetic and environmental factors act to constrain the lifespan of human diploid cells in culture is open to experiment. In the present study we have evaluated one important ambient factor of the environment, the oxygen concentration, on the growth and lifespan of two human diploid cell strains, WI-38 and IMR-90. Our results show that growth at oxygen concentrations less than 20% is enhanced and that an extension of about 25% of the lifespan of both cell types was realised by long term cultivation under 10% oxygen. This indicates that the potential for long term growth in culture of normal diploid cells may be appreciably enhanced by more exact control of environmental factors.