The costs of providing access to serial literature in four university research libraries were examined in this study, and a methodology was developed for comparing borrowing costs with the costs of acquisition, cataloging, maintenance, and circulation. Mathematical models are provided by which any library can determine at what frequency of use of a serial title it becomes less expensive to acquire a photocopy of an article from another library when needed than to subscribe to and maintain a file of the title. Cost data from the four libraries, when inserted into the models, indicate a strong case for borrowing low demand serial items. Study findings must be qualified because the four libraries are not a random sample of all research libraries, and no value was placed on having a collection available for browsing or on the shortened access time involved with local ownership. It is concluded that in order to give the research library a choice between borrowing and owning little used serials a national lending library system for serial literature needs to be developed. Appendixes include: details on the library surveys, mathematical analyses performed for the study, suggestions for a lending library system, and cost models for a specific serial title. (JB)