A disposable assembly for a handheld gas plasma tissue treatment instrument has a heat-resistant dielectric tube (516) which is housed in an instrument nose casing (517). The casing has a latch feature permitting releasable attachment of the assembly to the body of an instrument handpiece (501). The latter contains inner and outer electrodes (506,507) which, when the disposable assembly is attached to the handpiece body, lie respectively inside and outside the dielectric tube (516). A field focussing element (521) is incorporated in the tube (516) and is located in registry with the end of the inner electrode (506) when the assembly is attached to the handpiece body.
