Chinese hamster V79 cells and mouse EMT6 [mammary tumor] cells were made extremely hypoxic with a chamber technique that relies on passive gas exchange rather than respiration assisted O2 depletion. Cells in these chambers were treated with heat (43.degree. C) or heat plus X-irradiation, and colony forming ability was determined. Hyperthermia kills exponentially growing V79 cells more efficiently than plateau phase cells; kills euoxic and extremely hypoxic cells equally and radiosensitizes euoxic and extremely hypoxic cells to the same degree, leaving the O2 enhancement ratio unchanged.