The growth environment in an animal body differs considerably from one part of the body to another. This is especially true for the partial pressure of oxygen; it ranges from 2.6% to 21%. The majority of mammalian cells, both normal and tumor, actually live in the low end (below 5.2%) of this wide range of physiologic oxygen tension. In this study, the effects of pretreatment growth conditions on the sensitivity of tumor cells to various cytotoxic agents were investigated using murine Ehrlich ascites tumor cells grown in two different normoxic environments. The tumor cells adapted to grow in the peritoneal cavity of mice were found to be more sensitive to ionizing radiation, oxygen loxicity, doxorubicin and bleomycin than tumor cells adapted to grow in vitro. However, there was no difference in their sensitivity to 5-fluorouracil. One obvious difference between these two growth environments is oxygen tension; it is between 2.6% to 5.2% (20 to 40 mmHg) for the peritoneal cavity and 21% (159 mmHg) for the regular tissue culture. To investigate the role of oxygen tension, tumor cells from the peritoneal cavity were grown in tissue culture having either 21% O2 or 4% O2 in the gas phase. Within 4 days, tumor cells that were exposed to 21% O2, but not to 4% O2, in vitro gradually became as resistant to cytotoxic agents as the tumor cells continuously cultured in vitro under 21% O2. It appears that the adaptation of tumor cells to different environments having different partial pressure of oxygen alters their sensitivity not only to oxygen toxicity but also to ionizing radiation and other cytotoxic agents that damage or kill cells by generating free radicals. It is concluded that the data obtained from cells that are adapted to grow under 21% oxygen may not always be applicable to cells that are grown under low physiologic oxygen tensions.