A microcomputer-based user interface was developed and programmed for the library computer systems at the University of Illinois. Designed to provide user-friendly access to the two components of the online catalog on the library's IBM 3081 mainframe computer, the interface program resides on the IBM PC and queries the user in natural written English. System search commands--which are different for the two components--are then formulated internally by the interface, relieving the user of having to learn two distinct command languages. This description of the interface outlines procedures for conducting author and title, title, author, call number, International Standard Book Number, International Standard Serial Number, and subject searches. The implementation and problems of screen layout, natural language queries, subject searches, and computer programming are discussed in detail. (GL)