Several colonies of rats and mice have been reared totally free of viable microorganisms, some for over 10 years. A variety of other species including monkeys, dogs, sheep and swine have been maintained under the same conditions for shorter periods. This operation requires more precise control of microbic contamination than can be achieved by conventional techniques. The necessary control was realized by the use of apparatus termed "isolators". Obtaining sterility is a biological problem but its maintenance is an engineering problem. The gnotobiotic animal has been shown to be more sensitive to some microbic contamination than in vitro culture media. Apparently complex spacecraft cannot be sterilized effectively by postassembly treatment alone. Therefore, sterile subassemblies must be made as required with final assembly in a sterile environment to avoid trapping viable organisms. This can be accomplished within isolators since there appears to be no size limitations to the apparatus developed for biological and medical applications. Repairs and testing procedures under sterile conditions are possible with the isolator system. Fabrication of spacecraft within closed sterile isolators should present no major engineering problems provided all of the components are sterilizable.