A cost study was undertaken of the costs incurred by, or allocable to, the Interlibrary Lending (ILL) section of the UCLA Pacific Southwest Regional Medical Library Service (PSRMLS) from June 1, 1976 through May 31, 1977 for filled and unfilled book and journal requests. Mathematical methods were used to establish accurate values of total direct salaries, direct salaries applicable to each type of request, indirect salaries, direct materials applicable to each type of request, indirect materials, and number of each type of request. Cost estimates for interlibrary lending of the PSRMLS, based on Westat's overhead rate (from "A Study of the Characteristics, Costs, and Magnitude of Interlibrary Loans in Academic Libraries,") were compared to results of this study. The study concluded that the great disparity between the PSRMLS overhead rate of 170.8 percent and the Westat overhead rate of 50 percent tends to confirm that Westat omitted some overhead cost categories and/or miscategorized some individual salaries as direct. Cost underestimation of this magnitude could cause serious budget and plan miscalculation. Sections providing detailed information about the PSRMLS, derivations of estimated direct and overhead ILL costs, and data tables are included. (KP)