A novel model was developed to characterize the time-varying clearance of recombinant interleukin-2 (IL-2). Sixty-eight patients with human immunodeficiency virus infection received 83 cycles of IL-2 either by continuous infusion or by subcutaneous injection for 5 days. IL-2 concentrations after intravenous infusions peaked at 24 hours and then declined by 55% to 78% during the remainder of the infusion. Soluble IL-2 receptors increased greater than 10-fold before gradually returning to baseline. Subcutaneous administration showed a dose-dependent decrease in area under the concentration-time curve (AUC) between days 1 and 5. A model was developed in 9 patients who had IL-2 concentrations and soluble IL-2 receptors determined by ELISA. Concentrations were fitted by an indirect stimulatory pharmacodynamic model. An additional 59 patients with only IL-2 concentrations were fitted to a simplified empiric model. Both models provided an overall r2 of 0.99 for the plot of observed versus fitted concentrations. The time-dependent increase in IL-2 clearance, likely receptor-mediated, was well described with use of an indirect-effects pharmacokinetic-pharmacodynamic model.