OBJECTIVE It is well known that the radiosensitivity of tumor cells can be significantly reduced under hypoxic conditions. However, most of the reports in the literature refer to an experimental setup in which the supply of oxygen is kept low for a short period of time only. In tumors, chronic hypoxia would seem to be the more typical situation, because of an insufficient vascularization and the limited diffusion of oxygen into the tissue. Under such conditions, certain changes in the proliferation patterns of tumor cells, in which the cell cycle checkpoint protein p53 seems to play a role, have been shown to occur. We therefore decided to study radiosensitivity and cell cycle progression under conditions of chronic hypoxia in several human tumor cell lines differing in their p53 status. METHODS Four human tumor cell lines (melanomas Be11 and MeWo and squamous carcinomas 4197 and 4451) were incubated for 3 h, 24 h, and 72 h under either oxic or hypoxic conditions and subsequently exposed to graded doses of X-rays. In some cases, cells were kept under hypoxia for the same periods of time, but then reoxygenated immediately before irradiation. Cell survival was assessed with the usual colony formation assay, and cell cycle distributions were determined by two-parameter flow cytometry after labeling with bromodeoxyuridine (BrdU). RESULTS As expected, the oxygen enhancement ratio at 3 h was 2.0 or more in all cases. Differences, however, became evident with longer incubation times. At 24 h, the sensitivity of cells kept under hypoxic conditions both before and during irradiation was practically unchanged with cell lines Be11, 4197, and 4451, but clearly increased with MeWo. This resulted in an oxygen enhancement ratio of only 1.1 for the latter cell line when the sensitivity of aerated cells was used as reference. Cells kept under hypoxia for 24 h and reoxygenated shortly before irradiation, however, also showed an increase in sensitivity, so that the oxygen enhancement ratio based on differences in irradiation atmosphere alone was still around 2.0. At 72 h, the two p53 wild-type cell lines were not available for experiments, because they quickly degenerated under hypoxic conditions. Both mutant cell lines now showed similar results, the sensitivity being increased with irradiation under continued hypoxia as well as after reoxygenation. The oxygen enhancement ratios with reference to aerated cells were 1.3 and 1.5 for MeWo and 4451, respectively. Flow cytometric measurements after labeling with BrdU revealed that in all cell lines, the fraction of active S-phase cells during incubation tended to decrease under hypoxic conditions. Only in the p53 mutant cell lines, however, was this accompanied by an increase of the percentage of S-phase cells that were not actively incorporating BrdU. CONCLUSIONS It is suggested that these quiescent cells in the S-phase compartment develop because of a general breakdown of cellular energy metabolism. In the p53 mutant cells, this may lead to a cessation of cell cycle progression in all phases alike, because checkpoint control has been lost; p53 wild-type cells, on the other hand, settle down preferentially in G(1) under the same conditions. Independently of the p53 status, however, energy depletion may be the cause of a decreased ability to cope with radiation damage and thus the cause of the observed increase in radiosensitivity. This would become more easily apparent in the p53 mutant cell lines, because they are less sensitive than the p53 wild types to hypoxia as such.