A split dose method has been presented for the determination of the extrapolation number (with confidence limits) of the second dose survival curve following a given first or conditioning dose of radiation. Application of this method to published data (for mouse dermal and jejunal mucosal epithelia), where the slopes of the first and second dose curves were accurately measured but the extrapolation numbers were unknown, has yielded well determined values of the extrapolation number of the second dose survival curve. The case where direct measurement of the survival curve slopes is not possible has also been considered, and as an illustration the method has been applied to planarian stem cells.

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