A test method for measuring the minimum concentration to support an intraluminal flame is described. An oxidizer is flowed through a plastic tube, and an ignition source is applied to the free end of the tube. If the oxygen concentration is sufficient, a flame propagates along the inner surface of the tube, termed an intraluminal flame. The minimum concentration of oxygen, or limiting oxygen index (LOI), that will support such a flame is determined. Results are reported for polyvinyl chloride tubing and compared to those using the standard ASTM D 2863, the candle-type flammability test. In the intraluminal test, the LOI with helium as the diluent gas is lower than that with nitrogen as the diluent, in contrast to results using ASTM D 2863. The effects of buoyancy are reduced in the intraluminal test, and the results may therefore be more applicable to low gravity environments.