A mathematical model was used to consider alternative request routing policies for use in the Illinois Library and Information Network. Given data on interlibrary loan demands, the probability of request success, processing and delivery times for various libraries, and a network request routing policy, the model predicted the probability of satisfying a request, the average delay in receipt of the desired item, the cost, and processing loads. Comparing resource centers in the network revealed that Illinois State Library (ISU) and University of Illinois Libraries (UOI) are superior to Chicago Public Library (CPL) and Southern Illinois University (SIU) in terms of probability of request success and average processing times. When CPL and SIU were removed from the model, performance was not significantly degraded. While available data were too limited to make a strong recommendation for removing CPL and SIU from the network, a careful data collection procedure should be pursued to study the network and perhaps at some time in the future to automate data collection. (CH)