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A problem for radiobiology, radiation protection and treatment planning is the calculation of RBE as a function of the delivered dose when this dose is a result of a mixture of low and high LET radiations, as for neutron and pion beams. Track structure theory (Katz and Sharma, 1975) has recently been used to produce an algorithm for calculating survival, RBE, and OER as a function of dose in such environments. This simple formulation is based on the concept of irradiation equivalence (Katz et al., 1972) through which the effect of a mixed radiation environment is calculated as if it were due to a combination of equivalent track segment bombardments representing the low LET component (e.g. X-rays, electrons) and one or more high LET components. The equivalent irradiation for any one component of a mixed environment is the track segment bombardment which yields the same survival at a given dose as the original component. Additionally, the values of Πi and Πγ (the probabilities for survival at the same dose in the ion-kill and gamma-kill modes) for the component must be equal to those of the track segment bombardment. The agreement of theoretical calculations with experimental observation (Railton et al., 1974 & 1975) for CHO cells when sequentially radiated with neutrons and gamma rays (Katz and Sharma, 1974) and the recent calculations of RBE-dose relations for neutrons and pions lend support to the use of this algorithm.