The effects of different partial pressures of oxygen on the growth of hamster embryo and hamster tumor cells in culture were studied. Hamster embryo cells grown in medium with calf serum became established as continous cell lines in 1 to 3% O2 but degenerated in 20, 50, or 97% O2. The same cells grown in medium with fetal calf serum became established in 1 to 3 or 20% O2 and degenerated in 50 or 97% O2. Hamster embryo cells grown in medium with fetal calf serum in 20% O2 were less sensitive to O2 toxicity after 119 than after 46 culture days. Treatment of secondary or tertiary cultures of hamster embryo cells grown in medium with calf serum in 20% O2 with chemical carcinogens facilitated their establishment and increased their resistance to O2 toxicity. Cells that developed into established lines and cells that died became heteroploid during the first few weeks of their growth in culture. Lines of carcinogen-treated and untreated cells became tumorigenic in hamsters. Cells from tumors grew permanently in medium with fetal calf or calf serum in 1 to 3 or 20% O2, and were very resistant to higher O2 concentrations. Differences in growth rate and O2 consumption of hamster embryo and hamster tumor cells changed the concentration of O2 calculated to occur at the cell-fluid interface for a given concentration in the gas phase. Such changes may have been partly responsible for the observed differences in O2 toxicity.