Dependence of the survival curves of Burkitt lymphoma cells, which were featured by their small n or Dq values, on linear energy transfer (LET) obtained for different quality of radiation was revealed markedly in the change of D0 value, together with a small change in n value. Relative biological effectiveness (RBE) compared with Dq, n and D37 values of Burkitt lymphoma cells for high LET radiation was smaller than that of other cell lines. This finding supports the hypothesis that in Burkitt lymphoma cells the recovery capacity from sublethal damage (Dq) is so small even after low LET irradiation that LET does not modify the suppression of recovery. Similar survival curves with n value closely equal to 1 were obtained for four different mammalian cell lines (Burkitt lymphoma p3HR- 1, human melanoma HMV, HeLa-S3, and L5178Y) after 2 MeV neutron irradiation. This fact may suggest that the radiation which has an LET value at which n value of the survival curve is to be 1 will be optimum for therapeutic purpose to the radioresistant tumors.