[en] The Withers intestinal crypt clonogenic cell assay has been used to assess the relative effectiveness of high-energy heavy ions for cell killing in this system. The radiation modalities used are 228 MeV/amu helium ions and 400 to 670 meV/amu carbon, neon, and argon ions. These high-LET particle beams have been examined and their effect on both the shoulder of the cell survival curve and the region of exponential cell killing has been examined. All ions except helium are significantly more effective than 225-kVp x rays. There is both an effect on the shoulder, evidenced in a narrowing of this region, and an effect on the exponential region where the D₀ is reduced. This latter finding is in sharp distinction to the effects of neutron beams, where the shoulder is reduced, but the D₀ is unaffected. One of the most significant observations made in these studies is that LET alone does not predict the effectiveness of the various ions. Large differences in effectiveness are seen as a function of atomic number of the particle, even though the LET is held approximately constant