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The proposed environmental release of genetically engineereed microorganisms (GEMs) presents unique problems in hazard assessment. Currently, there are two general models, which might serve as guidelines for studying the environmental fate of GEMs. The first is to use the release of other non-indigenous organisms as a model for GEM fate. This approach has the advantage of focussing on the living properties of GEMs and provides insight not only into acute toxicity of the GEMs and gene products, but also into their ability to proliferate in host tissues and pass their genes to indigenous species. Alternatively, GEM fate can be analyzed by comparing GEMs to organic chemicals for which well-defined models exist. In this case, a primary consideration is to determine the degree to which a GEM is adsorbed or unbound. Further, the factors which affect adsorption, e.g. pH, temperature and soil characteristics as well as the population dynamics of GEMs and their ecological impact, must be assessed. Regardless of which model is chosen, the ultimate challenge is to develop methods which permit the screening of environmental samples for the presence of GEMs, their genes and gene products. Whole organisms can be detected using viable cell counts on antibiotic-amended agar as well as by serological means. Genes (and gene transfer) can be screened by Southern blots; the presence of various gene products can be detected by bioassay or serology. Microcosms, incorporating several trophic levels, provide a means by which the direct and indirect, as well as the acute and sublethal, effects of a GEM can be gauged.