Factors affecting flow through blood administration sets in vitro were assessed under gravity-fed and pressurized conditions including an assessment of the influence of the intravenous (i.v.) cannula and Luer lock fitting. The fastest gravity-fed flow of 4.775 mL s−1 was obtained through the largest internal diameter (ID = 4.8mm) blood administration set. Flow through blood administration sets with ID = 3mm was 50% of this. Flow increased over base-line through all the administration sets when the i.v. cannula was removed (range 18–50%) and increased further over base-line when the Luer lock fitting was removed from the distal end (range 26–129%), indicating that these are rate-limiting steps in the system. The Y-type trauma set with the largest diameter tubing facilitated the fastest flow, although flow through all the Y-type trauma sets produced lower flow rates than the corresponding blood administration sets, which may reflect their relative increased length. The ideal blood administration set should have an internal diameter at least 4mm and be less than 170 cm in length.