To study macromolecular structure by electron microscopy, a highly extensible and object-oriented system has been developed for image analysis. This system is named ''Eos'' (Extensible and object-oriented system). The system described here supplies an environment with four types of supports: (i) a group of small tools for image analysis, (ii) tools for integration of small tools, such as ''Display2,'' (iii) tools for development, such as ''maketool,'' and (iv) object-oriented libraries for development of new tools. Using Eos, electron micrographs can be analyzed by small tools and integration tools. In addition, Eos can be used to develop new tools based on new ideas because development tool and object-oriented libraries are provided. The examples of implemented small tools for image analysis include three-dimensional reconstruction of objects with helical symmetry, cluster analysis, and contour expression. (C) 1996 Academic Press, Inc.