The effects of hyperthermia temperature and the presence of lanthanum on the release of Ehrlich's ascites cell molecules labelled with radiophosphorus have been studied. The leakage of intracellular molecules is in relationship with the temperature and the time of incubation. The phenomenon presents the characteristics of an increased passive diffusion induced by a thermotropic modification of the protein-lipid environment of the plasma membrane. The possible implication of this phenomenon on hyperthermic energy depletion and enhanced cytotoxicity of chemotherapeutics with poor cell penetration is discussed.