In addition to the increased synthesis of the classical heat-shock proteins (28,000, 71,000, 73,000, 90,000 and 100,000 Mr polypeptides) there is also an increase of thrombospondin in the growth medium of endothelial cells exposed to hyperthermia. The effect of a monoclonal antibody to thrombospondin on the recovery of endothelial cells from hyperthermia as it relates to cytoskeletal organization and cell spreading was assessed. The antibody interacts with the heparin-binding domain of thrombospondin in the extracellular matrix of cells. We report that during recovery from thermal insult at 37 degrees C, intermediate filaments, stress fibres and microtubules show distinct time-recovery characteristics in bovine aortic endothelial cells; that in the presence of this antibody the cytoskeleton is notably altered; that this antibody causes retraction of endothelial cell processes; and that the recovery of the cytoskeleton in endothelial cells exposed to hyperthermia is prevented by the thrombospondin antibody in the time frame examined. Our data suggest that the recovery of cells from heat shock requires the integrity of thrombospondin and its interactions.