[en] A mechanistic state vector model (MSV model) has been developed to describe the interaction of ionizing radiation with cells and the consequences thereof. The MSV model considers the multifactored, nonlinear, and probabilistic nature of the interacting processes. The model is kinetic in nature and considers intracellular transitions involving the production, correction, and confirmation of radiation induced damage. Some important implications of the MSV model for systems irradiated at constant dose rates are as follows: Systems that accumulate sublethal damage should show an increasing cellular mortality rate with increasing dose of low LET radiation, if the efficiency of the radiation in killing cells increases as the amount of sublethal damage increases. However, for some cases of high LET irradiations, the cellular mortality rate may decrease with increasing dose. The shape of a radiation dose-response survival curve is determined by the cellular mortality rate profile. The absence of a shoulder in a dose-response survival curve is not necessarily an indication that repair machinery is inoperative, but is a consequence of an invariant cellular mortality rate. The extrapolation number, D/sub q/, and D/sub O/ may be dose-rate dependent in some cases. Increasing the dose rate may in some cases afford a dose sparing effect. Target-hit models are in fact special cases of a more general kinetic model. Purely dose-dependent target-hit models can not account for dose rate effects, especially at low dose rates; nor can they account for post irradiation transitions. (U.S.)