HeLa S-3 cells in culture were used to determine the biological effectiveness of a 20-MeV electron beam at the depths of the relative 100% and 25% dose in a polystyrene phantom. It was found that the survival curves at the two different depths were similar and that the extent of the recovery from sublethal irradiation was not significantly different. The clonal size distribution revealed no qualitative differences between the two depths. It was thus concluded that the biological effectiveness of 20-MeV electrons at the two depths was not significantly different. The variability in the biological effectiveness observed in some reports with depth may have resulted from uncertainties in dosimetry or changes in environmental conditions.

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