Synthetic polyamide comprising one or more of the following compounds: a) a compound having at least one sterically hindered amino group, [hereinafter referred to as compounds a)]; or b) a trimesinic acid trialkylamide, at least one alkyl group of which bears at least one free amino and/or alkylamino group [hereinafter referred to as compounds b)]; or c) a trialkylamino-substituted triazine, at least one alkyl group of which carries a free amino and/or alkylamino group [hereinafter referred to as compounds c)]; or d) a dyestuff that is free of sulphonic acid groups and contains at least one group that is reactive to a hydroxy and/or amino group [hereinafter referred to as compounds d)]; or e) a synthetic polyamide containing at least one group that is reactive with a hydroxy and/or amino group [hereinafter referred to as compounds e)].