Samples of asexual spores of a two-component heterokaryon of Neurospora crassa collected on Millipore filters were flown on Biosatellite II in September 1967 to evaluate the genetic effects of radiation in combination with weightlessness. Preliminary genetic analyses for cellular inactivation and the overall induction of recessive lethal mutations in the ad-3 region reported previously [F. J. de Serres and Webber, B. B., Bioscience 18, 590-595 (1968)] indicated that the effects of radiation under weightlessness were not different from those in earth-based experiments. The genetic analysis of the ad-3 mutations in the flight and ground control samples is now complete, and comparisons have been made between the dose-response curves for the different classes of ad-3 mutations, showing that there is no effect of weightlessness on the spectrum of radiation-induced recessive lethal mutations. These data confirm the negative results obtained with dormant spores flown on Gemini XI [F. J. de Serres, I. R. Miller, D. B. Smith, S. Kondo, and M. A Bender, Radiation Res. 39, 436-444 (1969), and F. J. de Serres and D. B. Smith, Radiation Res., in press].

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