Knowledge-based search tactics are discussed in terms of their role in the functioning of a semantically-based search system for bibliographic information retrieval. This prototype system, EP-X, actively assists users in defining or refining their topics of interest. It does so by applying search tactics, to a knowledge-base describing topics in a particular domain and a database describing the contents of individual documents. arises because: 1. The searcher needs to learn more about the topic area; 2. The searcher is having difficulty expressing her interest clearly; 3. The searcher needs to broaden or narrow her topic because too many or too few documents have been retrieved. Over the past five years, we have been studying ways to assist such information seekers and Krawczak, 1989). This paper summarizes our results and current conclusions about the design of computerized search intermediary systems. Particular attention is given to the idea that, in a semantically-based search system, knowledge-based search tactics can be applied to help the searcher explore a topic area. DEVELOPMENT OF A SEMANTICALLY-BASED SEARCH SYSTEM As part of the initial development of EP-X, we studied intermediaries who were also experienced indexers at Chemical Abstracts Service. Thus, in addition to having experience as intermediaries, they were very knowledgeable about a particular field of chemistry and about the practices and policies followed at Chemical Abstracts when indexing documents in that field. Based on several informal studies of these intermediaries, we concluded that there were four critical aspects of the interactions that we had observed: language fact retrieval, ACM Transactions on Database Systems.M.H. EP-X: A knowledge-based system to aid in searches of the environmental pollution literature.