In connection with planetary quarantine, we have been studying the survival rates of nine species of terrestrial microorganisms (viruses, bacteria, yeasts, fungi, etc.) under simulated interstellar conditions. If common terrestrial microorganisms cannot survive in space even for short periods, we can greatly reduce expenditure for sterilizing space probes. The interstellar environment in the solar system has been simulated by low temperature, high vacuum (77 k, 4 x 10(-6) torr), and protons irradiation from a Van de Graaff generator. After exposure to a barrage of protons corresponding to about 250 years of irradiation in solar space, Tobacco mosaic virus, Bacillus subtilis spores, Aspergillus niger spores and Clostridiun mangenoti spores showed survival rates of 82%, 45%, 28%, and 25%, respectively. Furthermore. pathogenic Candida albicans showed 7% survival after irradiation corresponding to about 60 years in space.