Many patients given murine monoclonal antibody become immune to these proteins. Important factors in the development of immunity are (1) the patient's ability to become immune and (2) whether whole immunoglobulin or Fab fragment was used. Circulating anti-murine antibodies increase the murine antibody plasma clearance and the liver or spleen uptake while reducing tumor targeting. Clearance rates and radioassay measured levels of anti-murine antibodies are related to serum murine/anti-murine antibody complexes. Unlabeled murine antibody displaces radiolabeled antibody from these complexes.