Hyperthermia is a therapeutic modality under investigation for its ability to increase absolute levels of tumor uptake of radiolabeled monoclonal antibodies (MAbs). We have investigated whether hyperthermia may affect the binding parameters of MAbs. The effects of clinically relevant levels of hyperthermia on the kinetic binding parameters were investigated for 81C6, an antibody undergoing Phase I/II clinical trials for the treatment of brain tumors and neoplastic meningitis. No obvious effects of temperature in either the association or dissociation rate constants, nor in the equilibrium constants, were apparent between 37 degrees and 45 degrees C. The improved binding stability of the bivalent form of the MAb was apparent when compared with its monovalent Fab fragment.