"A Virtual union catalog is a possible alternative to the centralized database of distributed resources found in many library systems. Such a catalog would not be maintained in a single location but would be created in real time by searching each local campus or affiliate library’s catalog through the Z39.50 protocol. This would eliminate the redundancy of record storage as well as the expense of loading and maintaining access to the central catalog. This article describes a test implementation of a virtual union catalog for the University of California system. It describes some of the differences between the virtual catalog and the existing, centralized union catalog (MELVYL). The research described in the paper suggests enhancements that must be made if the virtual union catalog is to become a reasonable service alternative to the MELVYL® catalog. "