A model is presented for the statistical analysis of survival curves and mutant frequency curves for a hybrid cell system. The derivation of the model is given in the Appendix, and depends on simple assumptions about the distribution of insults, their repair, and the loss of a marker that is not rescued. A single formula (5) is found which relates a survival curve to the mutant frequency curve, i.e., the response curve for production of mutants per 105 survivors induced by a mutagen. The analysis is applied to loss of the at gene in AL-J1 hybrid cells submitted to Cesium γ-rays. Previous experimental data using X-rays was reported by Waldren et al. (1986: Proc. Natl. Acad. Sci, USA 83, 4839.) Also, a derived formula (10), which predicts the probability that in a surviving cell a marker is lost and not rescued, will form the basis for testing the validity of the model in the future using new experimental data.