Variables studied in typical cellular radiation biology experiments are cell killing, mutagenesis, transformation to malignancy, heritable damage, and DNA damage and repair. Dose response curves for cells exposed to low-LET radiations and some high LET radiations are well known. The low-LET dose rate in low earth orbit is roughly 1.0 mSv/day, the heavy-ion (Z>2) flux is about 1.0 particle/cm2-s corresponding to about 0.3 mSv/day, and the integrated neutron flux is roughly 2 neutrons/cm2-s corresponding to 0.012 mGy/d or, assuming a QF of 10, 0.12 mSv/d. Published dose-response curves were used to estimate the probability that a mammalian cell will be affected by each of the above types of damage. As a general approximation the exposure of an experimental cell population to the space radiation environment for 100 days will result in the following probabilities of damage per cell: cell killing based on clonogenicity 0.02, mutagenesis per locus based on phenotype analysis 1 x 10(-6), point mutation induction 2 x 10(-8) per locus, malignant transformation in vitro based on colony morphology 1.2 x 10(-5), heritable damage based on colony size 0.02, and induced DNA double-strand breaks based on fragment analysis by electrophoresis 3.5/cell or 0.26/cell after repair. Most of these figures are accurate to within a factor of 2. Thus the spaceflight radiation environment has essentially undetectable impact on typical cell biology experiments unless experimental goals involve the precise measurement of one of the above end-points. Other in vitro end-points, such as tissue morphogenesis and cell differentiation, are expected to be similarly unaffected by the spaceflight radiation environment.