[en] We have investigated the number and distribution of radiolesions produced by x rays and accelerated atomic nuclei in mammalian cells. This is a difficult task because the initial lesions in DNA are usually of atomic dimensions. During most of the cell division cycle, we cannot observe DNA in the cell nucleus without killing the cells and highly denaturing the genetic material. Premature chromosome condensation make chromatin lesions visible. By use of these lesions are produced in a random Poisson distribution after doses of x rays, but exposures to accelerated heavy ions result in distributions of lesions that were very different from Poisson. 4 refs., 4 figs