It is shown that loading ultra-high molecular weight polyethylene (UHMW-PE) fibres up to about 50% of their theoretical strength causes bond scission to occur as corroborated from the number of radicals monitored. In addition, a creep test on UHMW-PE fibres also showed an ongoing production of radicals in the polymer. Although precise quantification of the radical concentration is difficult, a first estimate shows that there is a significant number (of the order of 4 x 10(13) spins) compared with the total number of polymer chains (10(14)) that run through the cross-section of the PE rope employed in the experiments.