[en] Multiple biopsies were obtained from eight patients with malignant tumors during a course of fractionated radiotherapy. An analysis of the total 2-nitroimidazole concentration and a histological examination of the biopsy sample was done on material which appeared macroscopically similar. The parts of the tumor biopsies showing necrosis were found to contain much lower concentrations of nitroimidazole when compared with those found in the samples with well-vascularized and apparently viable tumor. During the course of irradiation changes in misonidazole concentrations were not found when similar histological material was examined. The low values of misonidazole in necrotic regions in human tumors may partly account for the wide variation of misonidazole concentrations reported in the literature (percent tumor/plasma, mean 62.9 +- 34.4 STD). The clinical significance of the microscopic distribution of misonidazole in tumors in unknown but further work in this area may aid in predicting the usefulness of future radiosensitizers