[en] The non-clonogenic MTT assay, based on the reduction of a tetrazolium salt to a purple formazan precipitate by living cells, was modified and a new procedure of analysis proposed. X-ray survival curves were generated for V79, CaSki, WiDr and HeLa cells using the non-clonogenic and a standard clonogenic assay. The described assay is a feasible and reproducible technique for determination of cellular survival, which may be able to incorporate progression delay. The equivalence to a clonogenic survival assay could be proven. (author)