Some of our earliest studies on recovering viruses from foods had been sponsored by the US Air Force School of Aerospace Medicine (Herrmann and Cliver 1968a) , which explains the inclusion of two freeze-dried foods that had been developed for astronauts (Herrmann and Cliver 1968b ). Subsequently, we were contacted by the life support groups of the US National Aeronautics and Space Administration (NASA) regarding both astronaut foods and the spacecraft water supply. Electricity aboard spacecraft was generated by hydrogen-oxygen fuel cells, so that the water produced as a byproduct was available for drinking and for rehydration of foods that had been freeze-dried to save weight. The food studies were largely precautionary-NASA had a great many ''what-if'' concerns at that time. Low-moisture foods studied included bacon squares, beef bites, cheese sandwiches, spaghetti with meat sauce, and banana pudding . Viruses tested included influenza virus type A (strain PR8), parainfluenza virus type 3 (strain SF-4), reovirus type 1 (strain Lang), EC6 (strain D'Amori), and various polioviruses (some in the feces of infants who had received the trivalent oral polio vaccine). The influenza, parainfluenza, and reoviruses persisted for B3 days in inoculated low-moisture foods; whereas the enteroviruses persisted [2 weeks at room temperature, and [2 months in the refrigerator. Various other temperature-storage regimes were also evaluated; poliovirus of fecal or cell-culture origin behaved similarly. Fecal poliovirus was inactivated 10 -2 during freeze drying in cream-style sweetcorn, but the residual virus persisted with little loss during 15 weeks' storage at 5°C. The water-system studies were inspired by a technological problem. Although the water produced by the hydrogen-oxygen fuel cells had\1 ppm total solids, it was mixed with other water (such as condensate from inside the space suits) and stored in a reservoir lined with a polymer bladder. Evidently, the ultrapure water leached enough solute (either plasticizer or unreacted monomer) to support the growth of the so-called distilled-water bacteria (e.g., Pseudomonas aeruginosa). These created a nuisance; and since check valves were unreliable in a weightless environment, other potential modes of water contamination were envisioned. NASA had commissioned development of an electrolytic silver-ion generator that was intended to decontaminate the water; it was demonstrably capable of generating silver ions in water, but the antimicrobial effectiveness of these ions had not been tested. We were asked to determine the effectiveness of very low silver-ion levels against both distilled-water and pathogenic bacteria, as well as viruses. This required a team, in that we needed help measuring the low levels of silver ions and a more experienced bacteriologist than I. Silver-ion levels in the working range of 50-250 ppb were estimated by a technique called neutron activation analysis (NAA) by Dr. Wesley K. Foell of the UW Department of Nuclear Engineering. Quantification of silver by NAA turned out to be something of an art form; we were not confident enough of the measurements to submit our findings for journal publication. Bacteriological results, reported by Dr. John M. Goepfert of the UW FRI, will not be described here, except to say that substantial kills were obtained at the higher end of the silver-ion working range. Among the viruses tested, only vaccinia appeared to be completely resistant to silver ions. The viruses tested, in order of increasing silver sensitivity, were influenza type A, several enteroviruses, reovirus type 1, and rhinovirus type 1A. The time dimension for inactivation of these viruses ranged from days to minutes. The rate of inactivation was similar, whether the silver ions had been added by an electrolytic generator or as a soluble salt. The concentration of silver (in the range of 50-250 ppb) was not always the principal rate-limiting factor in inactivation of the virus. We were not able to measure the uptake of silver by the virus particles. Extremely pure water was not necessary for viruses to be inactivated by silver ions. However, feces (\1 ppm, or a dialyzable component of feces) were extremely effective in preventing silver inactivation of enteroviruses and, to a lesser extent, reovirus. Respiratory mucus did not show this sparing effect.