The invention is related to novel polycaprolactam filaments and fibres capable of being deeply dyed with acid dyes to give dye finishes with extreme fastness to light, characterized by the fact that they consist of physical mixtures of unmodified polycaprolactam and a basic polycaprolactam containing structural units corresponding to the formula: ##STR1## in which R represents an alkylene radical with 4 to 12 carbon atoms and x is an integer from 1 to 5, have relative solution viscosities of from 2.9 to 3.5 (as measured on solutions of 1 g of non-extracted polyamide in 100 ml of cresol at 25.degree. C) and contain free amino groups in a concentration range from 60 to 180 millivals per kg of polymer mixture.