Normal blood vessels are leakage proof, non-adherent to blood cell elements, and participate actively in directional blood flow. These properties rely on the shape of endothelial cells and the integrity of the endothelial cell monolayer. The often observed effects of tumour necrosis factor-alpha (TNF) and hyperthermia on tumour tissue are the disruption of blood flow and an increase of vascular permeability. These agents are also known to affect the cytoskeletal organization and the cytoskeleton-dependent cellular functions. We observed that TNF (100 U/ml for 60 min) or heat (43 degrees C for 60 min) treatment causes the collapse of actin filaments in human umbilical vein endothelial cells (HUVEC). The combined treatment of TNF and hyperthermia intensifies the change of shape and loss of actin filaments. However, these changes are reversible within 24 h. These transient changes may contribute to the dysfunction and increased leakage of the microvasculature in tumours during and after these treatments despite the fact that the viability determined by MTT assay did not show a significant interaction between TNF and hyperthermia. Radiation (5 Gy) and TNF interact to a lesser extent compared with heat and TNF on cell shape and actin filament organization in HUVEC. Heat or radiation treatment enhances the expression of ELAM-1 mRNA in HUVEC while TNF produces the strongest effect on ELAM-1 mRNA expression. Our study suggests that radiation and heat affect endothelial cells and their subsequent functions differently. Result of an interaction between heat and TNF on endothelial cells supports the common notion that the anti-tumour effect by heat plus TNF treatments may benefit due to the increased disruption of vasculature function in the tumour.