Space Station Freedom offers an opportunity to study in depth the long-term effects of microgravity on the basic biology of animals and plants, an area of great importance to the permanent presence of man in space. NASA has long recognized the significance of spaceflight research using non-human biospecimens in understanding these broad aspects of gravitational biology. To this end, plants and small animals have flown on manned and unmanned missions since 1965, including the highly successful Spacelab Life Sciences-1 (SLS-1) mission in June 1991.To minimize the potential for problems related to the inflight exchange of microbes between crew members and research biospecimens, NASA has established a policy of microbiological certification of animals designated to fly on NASA manned missions. The Human Research Policy and Procedures Committee at the Johnson Space Center administers this policy.Spacelab-3 (May 1985) and subsequent Shuttle flights involving animals have provided valuable experience with these microbiological requirements. The successful verification of the Research Animal Holding Facility (RAHF) on SLS-1 has demonstrated the ability to successfully house small animals on-orbit and has prompted a review of the current microbiological certification policy. It has become clear that quality standards and streamlined procedures must be adopted to efficiently accommodate increased numbers of flight research animals. This is especially true as the Space Station era begins and animals are continuously present aboard Freedom.This paper presents a brief history of the NASA microbiological certification program, its current status, and some suggestions for program improvement. In addition to increased efficiency and effectiveness of the certification process, program improvements may lead to more efficient on-orbit experiment and maintenance operations, reduced potential for microbiology-related launch delays and experiment constraints, and enhanced scientific return from Space Station Freedom.