[en] The lesions produced in the genetic material by chemical mutagens, on the one hand, and radiations, on the other, are very similar. In both cases, they are either lesions in DNA or changes in the bonds between this DNA and the proteins which surround it. The lesions are sufficiently similar to elicit, in both cases, the activity of the same repair systems. The similarity between chemical and radiation induced mutagenesis can be demonstrated by checking that a strain which is hyper-sensitive to radiation because it lacks some repair system, is also hyper-sensitive to most chemical mutagens. These similarities between the lesions suggest that one can establish an equivalence between the 'dose' of a chemical and a dose of radiation, on the basis of the effects produced on some biological systems of reference. Once such equivalence has been established, one could extrapolate the rules of radiation protection to protection against that chemical. Is this principle applicable, and under which conditions. What prerequisites must be fulfilled. The goal of this paper is to answer these questions