A general analysis technique is introduced for extracting in vivo cell survival equation parameters from observation of tissue damage following fractionated doses of radiation. This technique has been applied to a new set of mouse foot skin-reaction data and it has been possible to demonstrate that the parameters for an assumed survival equation can be determined independently of the arbitrary definition of the average skin reaction. Since all data points are used, no graphical data interpolation is required and the use of an arbitrary level of damage for isoeffect comparison is not necessary.