Cationic polyethylenimine (PEI) derived transfection reagent is optimized for the delivery of genes to fragile primary cells, such as human umbilical vein endothelial cells (HUVEC) and vascular endothelial cells of various origins. The jetPEI-HUVEC transfection reagent has demonstrated efficiencies as high as 50% with very little cytotoxicity. The reagent is able to deliver genes to HUVEC cells that are cultured in media with a range of serum concentrations. Gene expression reaches a maximum for HUVEC cells incubated for hours in medium containing 2% serum.