Part III is intended to serve as an index for diagnostic problem solving in situations for which infectious agents of mice and rats may be responsible. Three index categories, clinical signs, pathology, and research complications, have been listed in alphabetical order. When a problem suspected of being caused by an infectious agent is encountered, one or more of these lists should be consulted to identify quickly the most likely causative agent(s). One should then consult the information in Part II on the most likely candidate agents to narrow the list of possible causes and to devise further testing to make the definitive diagnosis. Once the precise cause(s) is known, specific corrective measures can be implemented.