The biological effects of ionizing radiation in space has been the subject of considerable concern even before humans actually began the exploration of space. Advances in Biotechnology now make it possible to monitor mutations in humans. Here we report the study of five Russian cosmonauts with considerable space flight experience and four trainees. Although the levels of mutation in these subjects were higher than observed in age-matched Western controls, no significant difference in either mutant frequency nor mutational specificity was observed. We conclude that Low Earth Orbit radiation does not produce detectable levels of mutation in human subjects.