A method for the direct treatment towards the specific sites of a disease is disclosed. This method is based on the delivery of proteins by catheterization to discrete blood vessel segments using genetically modified or normal cells or other vector systems. Endothelial cells expressing recombinant therapeutic agent or diagnostic proteins are situated on the walls of the blood vessel or in the tissue perfused by the vessel in a patient. This technique, provides for the transfer of cells or vectors and expression of recombinant genes in vivo and allows the introduction of proteins of therapeutic or diagnostic value for the treatment of diseases.