[en] Densely ionizing radiation which induces malignant transformations damages the cells only sublethally. In order to explain this paradox the threshold specific energy z₀ was introduced independent of LET, deposited in the cell nucleus. Exceedance of value z₀ would lead to the inactivation of cells. The distribution function F(z,D) is derived expressing the probability with which any value of specific energy less than or equal to z is achieved at dose D. The function is dependent on the size of the target and the quality of radiation. The possibility is discussed of inducing a malignant tumor in the cell by the direct effect of alpha radiation, and its explanation by introducing threshold energy z₀ which would mean that not every sublethal lesion must induce malignant cell transformation. (J.P.)