Tumor necrosis factor α (TNFα) appears to be the main mediator causing the systemic manifestations of sepsis and septic shock associated with edema (1 ,2 ). In vitro it augments movement of permeability tracers across endothelial cell monolayers (3 ,4 ). Herein we describe a method which permits quantification and assessment of changes in the integrity of endothelial monolayers by measuring transendothelial flux (luminal to subluminal) of tracer macromolecules in the absence of hydrostatic and oncotic pressure gradients. The system consists of two compartments separated by a polycarbonate filter and permits highly controlled conditions.