Photodynamic therapy (PDT) of tumors can create hypoxia when oxygen is depleted by photochemical consumption or the oxygen supply is compromised by microvascular damage. However, oxygen is a requirement for PDT, and hypoxia during illumination can lead to poorer tumor response. As such, sensitive methods of quantifying tumor oxygen and evaluating its distribution may help in the development and optimization of treatment protocols. In this study, the hypoxia marker EF3 [2-(2-nitroimidazol-1[H]-yl)-N-(3,3,3-trifluoropropyl)acetam ide] was used to evaluate the oxygenation of PDT-treated radiation-induced fibrosarcoma tumors. Tumor-bearing mice were administered Photofrin (5 mg/kg) 24 h before PDT illumination at 75 mW/cm2, 135 J/cm2 (30 min). EF3 (52 mg/kg) was injected either within 3 min before PDT illumination, with tumor excision at the conclusion of illumination, or within 3 min after illumination, with tumor excision 30 min later. Control animals received EF3 alone, EF3 plus Photofrin, or EF3 plus illumination. After tumor disaggregation, staining with a fluorochrome-conjugated monoclonal antibody, and flow cytometric analysis, control tumors demonstrated an averaged median fluorescence intensity (+/- SE) of 17.1 +/- 2.8. EF3 binding significantly (P = 0.007) increased during PDT to a median fluorescence intensity of 48.9 +/- 8.3. In the 30 min after PDT, EF3 binding returned to control levels (median, 18.3 +/- 3.3). To evaluate the oxygen concentrations corresponding to these fluorescence intensities, an in vitro standard curve was created based on the in vivo exposure conditions. From this curve, the oxygen tensions of tumors exposed to EF3 under control conditions, during PDT, or after PDT were calculated to be 3.1-5.3, 1.2-2.4, and 3.0-5.2 mm Hg, respectively. Detection of EF3 binding using a monoclonal antibody correlated well with direct detection of binding using a radioactive assay. EF3 binding was linear with drug incubation for times from 1.5 to 60 min. Overall, this work demonstrates that hypoxia during PDT illumination of radiation-induced fibrosarcoma tumors can be detected by the hypoxia marker EF3. Hypoxia during illumination can be labeled separately from that found before or after PDT. Tissue oxygen tensions corresponding to EF3 binding levels can be calculated.