Dynamic mechanical measurements were made with a low frequency (similar 1 c/s) torsion pendulum at temperatures from -170 to 150°C, on thin polytetrafluoroethylene (PTFE) sheet irradiated in vacuum at 23°C with doses up to 5 megarads of gamma radiation. Marked radiation-induced increases in shear modulus and decreases in amorphous and crystalline damping peaks were observed. Irradiation increased the density of the polymer, and X-ray and infra-red measurements indicated an increase in the overall crystallinity of low density PTFE. Low crystallinity material showed bigger changes in modulus, damping and density than highly crystalline PTFE, and this may be due to the lower mobility of excited species or free radicals in the more crystalline regions of the polymer. Irradiation reduced the tensile strength of the PTFE mainly by reducing the strain-hardening contribution to the strength. With ductile low crystallinity material where strain-hardening was marked, irradiation produced a 52% decrease in ductility and a 35% decrease in tensile strength. By contrast highly crystalline PTFE which was not so ductile and showed little strain-hardening initially, lost all ductility, but its tensile strength was little affected. The results suggest that although some chain scission occurs during irradiation, cross-linking is prominent and this together with the unusual increase in crystallinity makes the polymer more rigid and brittle. Specimens irradiated in air rapidly lost mechanical strength; changes in dynamic modulus and damping were similar to those resulting from vacuum irradiation, but increases in density and crystallinity of low density PTFE were greater when air was present during irradiation.