An injection site (12) for infusion of parenteral fluids has a pressure-actuated valve (20) and a split septum unit (24), which prevent reflux of blood into the assembly (10). The septum unit (24) includes a resilient split septum body (64) precompressed so that the septum body (64) is caused to protrude proximally (78) upon insertion of a cannula (16). Consequently, upon removal of the cannula (16), there is essentially no “drumming” or creation of friction-induced negative pressures sufficient to generate blood reflux. The preloaded septum body (64) also has its proximal surface (74) essentially flush and coplanar with the adjacent proximal end (66b) of the tubular septum holder (66) to enhance the cleanliness of the unit (24).