The WorldCat xISBN service, the OCLC service that supplies International Standard Book Numbers associated with individual intellectual works represented in the WorldCat database, is available for commercial and high-use applications. The WorldCat xISBN service, which began as an OCLC Research project, is a machine-to-machine service that supplies ISBNs and other information associated with an individual work in WorldCat, a database for discovery of items held in libraries. It has been a supported service, available at no charge for individual, non-commercial use, since February. It is now also being made available for commercial and high-use applications (more than 500 requests per day) via subscription. The xISBN service is designed to help users find a resource when an ISBN assigned to any printing or edition of the work is known. Users submit an ISBN to the service to return a list of related ISBNs and selected metadata. ISBNs are related to each other using librarian-cataloged bibliographic records in WorldCat together with an algorithm that implements the FRBR model for information objects that brings together multiple versions of a work. The FRBR model keeps WorldCat users from having to browse numerous records that represent many different manifestations of a book--such as different printings, hardcover or paperback editions, audiobooks or film versions, for example--and brings them together under one record. The WorldCat xISBN service is designed for web-enabled search applications, such as library catalogs and online booksellers, and based on associations made in the WorldCat database, xISBN enables an end user to link to information about other versions of a source work.