Using a tracer technique, loss of cells from perivascular and average tumor cells of the syngeneic mammary adenocarcinoma EO 771 in male C57Bl/6J mice may be measured in the living animal, by the use of 125-labelled 5-iodo-2'-deoxyuridine (125I-UdR). It was the purpose of this paper to compare measurements in vivo with those made in vitro following local 60Co-gamma irradiation in the absorbed dose range from 10 to 27.5 Gy, incorporation of radioactivity into DNA of tumor cells and activity loss from labelled tumor cells were measured externally by a special scintillation counter device. In addition, by injecting the vital dye "light green" into the mice the I-125-activity of the stained viable and unstained necrotic regions were separately measured for loss of activity following gamma irradiation. A comparison was made between radiation induced growth delay and the depression of 125I-UdR incorporation into DNA of the proliferating tumor cells. After local tumor irradiation with a dose of 27.5 Gy 60Co gamma rays an enhancement of the activity loss by 0.5% per hour was externally observed for the perivascular tumor cell population. A lower enhancement of 0.4% per hour was externally registered in the average tumor cell population. Both values were evaluated relative to sham-irradiated control tumors. The measurements on isolated tumors were in comparatively good agreement with the external values. The activity loss rate from the viable, euoxic tissue increased by 0.4% per hour after 27.5 Gy 60Co gamma rays and by 0.3% per hour in the average cell population, the latter representing a mixture of euoxic and hypoxic cells. The results demonstrate, that the external measurements are a good indicator for radiation effects under in vivo-conditions.