The purpose of gathering an international expert panel for the workshop was to discuss containment technologies and to design a mesocosm chamber for use in the development of ecology databases and the calibration of test protocols for genetically engineered microorganism (GEM) risk assessments. Data obtained from GEMs mesocosm testing would provide a critical knowledge base for regulation of environmental releases of GEMs. Workshop presentations covered the positive and negative design features of specific microcosm and mesocosm systems that are currently being used in biotechnology and other kinds of research. Information from these presentations was used as the basis for design of a hypothetical mesocosm chamber that would meet appropriate containment and environmental control needs. The mesocosm was designed to support regulatory initiatives. It was not conceived to serve as a standard procedural tool mandated by regulatory agencies for evaluating risk assessments.