Returning geological samples from Mars is the next logical step as part of an integrated exploration program and is the critical subsequent stage in the search for evidence of life on the red planet. Analysis of such samples on Earth will generally produce data of superior accuracy and precision than can be accomplished in situ on the Martian surface, and such data will be invaluable in providing ground truth for remotely sensed observations. However, obtaining meaningful data requires that the samples returned to Earth suffer little (or preferably no) compositional or morphological changes during collection on Mars, transit to Earth, entry into Earth's atmosphere, impact on the surface, and long-term curation. This paper presents recommendations from NASA's committee for Curation and Analysis Planning Team for Extraterrestrial Materials (CAPTEM) for preserving the integrity of Martian samples during a return mission. Potential contaminants include dead bacteria on the outbound spacecraft and materials that come into contact with the samples during collection and storage. Therefore it is recommended that the spacecraft be sterilized and cleaned at least to Pathfinder standards, with the components that interact with the samples having a higher degree of cleanliness. Furthermore, it is recommended that only pure, homogeneous materials should be used for components that come into contact with the samples. The adverse effects of temperature on the samples are explored, and it is demonstrated that heat sterilization or undue temperature increases in transit or reentry have the potential to destroy important mineralogical and geochemical information. Concomitant with this will be degassing of the samples and stable isotopic fractionation. Therefore it is recommended that samples be kept at 240 K or below during transit, reentry, and curation. It is further recommended that heat sterilization not be used, except in extreme circumstances, and other sterilization techniques, such as radiation, be explored. While the purpose of this paper is not to design a sample return mission, the recommendations presented here are intended to act as guidelines for returning Martian samples in their pristine state. In essence, any procedure or piece of equipment that has the potential to compromise the pristinity of Martian samples during the collection, transportation, or curation needs to be fully evaluated by a scientific committee with the requisite expertise prior to implementation. CAPTEM is willing to serve in such a capacity if requested by NASA. ¿ 2000 American Geophysical Union