The crew of a manned Mars mission will be unavoidably exposed to galactic cosmic ray (GCR) flux. If one employs conventional radiological health practices involving absorbed dose(D), dose-equivalents (H), and LET-depen-dent quality factors (Q)2, the Mars mission crew shielded by 2 g/cm All could receive about 0.7 Sv in a 460-day mission at solar minimum. However three-fourths of this dose-equivalent in free space is contributed by high LET heavy ions (Z ≥ 3) and target fragments with average Q of 10.3 and 20, respectively. Such high quality factors for these particles may be inappropriate. Moreover, in a 460-day mission less than half of the nuclei in the body of an astronaut will have been traversed by a single heavy particle. The entire concept of D/Q/H as applied to GCR must be reconsidered.