The high binding specificity that monoclonal antibodies exhibit has led to great interest in using them to target tumor-associated antigens. The antibody may be coupled to a radionuclide or cytotoxic drug to create a tumor-targeted reagent that can be used to identify sites of metastatic disease and/or deliver a lethal substance to the tumor cells. However, successful application of these compounds in a clinical setting has been hindered by a poor understanding of the factors that govern antibody accumulation in a tumor. We have used a finite element method to develop a pharmacokinetic model describing the uptake of systemically-administered antibody in an early, prevascular spherical tumor nodule embedded in normal tissue. The model incorporates such processes as plasma kinetics, transcapillary transport, interstitial diffusion, binding reactions, lymphatic clearance, and antigen internalization.