An intravenous injection site having a split septum assembly interfit with the site for the intravenous administration of fluids to a patient. The split septum assembly has a resilient and compressible split septum having an axially-formed slit for receipt of a blunt cannula, needle, or other medical device through said slit, a septum holder for receipt of the split septum, and a septum housing for receipt of the combined septum holder and split septum. The septum includes a body and a flange extending radially from the body. A projection extends from the flange. The septum is mounted within the septum holder, and the septum holder is mounted within the septum housing to provide axial compression of the flange. This axial compression presents a double hermetic seal that significantly minimizes or prevents proximal leakage of fluid through the slit or around the septum holder when the cannula is removed.