An improved injection site (12) for infusion of parenteral fluids and the like is provided, having a pressure-actuated valve (20) and a novel split septum unit (24), which effectively prevent reflux of blood into the assembly (10). The septum unit (24) includes a resilient split septum body (64) which is 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). The specialized well (46) and septum unit (24) afford a resilient seal between the periphery of the septum body (64) and the surface (50), and a separate hard-surface seal between the outer margin of the surface (50) and the septum holder (66).