Abstract: A method of leak testing a transport vessel for radioactive material is disclosed. Tracer gas (helium) and pressurizing driving gas (nitrogen), the latter in condensed form, are introduced into the transport vessel when in open condition. The tracer gas is introduced in a plastic bag which occupies a major part of the interior of the transport vessel the through which the tracer gas diffuses at a low rate. The liquid pressurizing gas is introduced in a smaller vessel. After the transport vessel has been sealed, the tracer gas diffuses out of the collapsing plastic bag and the pressurizing gas boils off resulting in a pressurized tracer gas atmosphere within the transport vessel. The transport vessel is now examined on the outside with respect to any tracer gas penetrating out through possible leaks in the transport vessel.