Human endothelial cells from umbilical cord vein were grown to confluence on glass coverslips placed in Petri di= shes, in the presence or absence of dexamethasone. After ten days in culture, the cell monolayers were washed and treated with 0,5% Triton X lOO in Hank's solution , The detergent-treated slides were then washed in Hank's and examined by indirect immunofluorescence for fibronectin utilizing monospecific antibodies. Basement membranes produced by dexamethasone-treated cells showed a

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... ore intense fibrillar network of extensively overlapping fluorescent fibers if compared with controls.The effect of these pre-formed basement membranes on cellular spreading was subsequently studied. Human endothelial cells , previously grown in culture medium with out steroids, were detached by trypsin-EDTA solution,washed and cultured on the slides covered with the extracel lular matrix pre-formed in the presence or absence of dexamethasone. The higher fibronectin content of basement membranes pre-formed in the presence of dexamethasone allowed a more rapid attachement and a better spreading of endothelial cells if compared with control pre-formed basement membranes.