This paper describes an analytic model of an interactive multiprogrammed computer system. The model accepts a multiple-user-class, transaction-oriented workload description and a system configuration description, and it produces predictions of resource utilizations, transaction rates, and average transaction response times. The solution method involves nearly complete decomposition, with a closed queuing network representing the multiprogrammed set. Asymptotic formulas are used to generate good initial guesses for an overall iterative scheme. Extensive validation results are presented.