The receptivity to dyes of fibres based on crystalline olefin polymers (e.g. polypropylene) and basic nitrogen-containing polymeric tinctorial modifiers, is improved by immersing the fibres in a solution of trimethylolpropane diallyl ether. Specified tinctorial modifiers are polymers of vinyl pyridines, condensation products of epichlorhydrin/piperazine/primary aliphatic amine, condensation products of dihalogenated hydrocarbon/diamine (optionally alkylated products), and basic polymers of epichlorohydrin. The fibres may contain solid dispersants for the tinctorial modifier, and listed are cetyl and stearyl alcohols, stearic and terephthalic acids, benzoin, furoin, vinyl stearate, stearic esters of glycerol, monoethanolamine stearate, N-diethanol lauramide, aliphatic amines, condensates of ethylene oxide with alcohols, amines or phenols, polystearamide, polyacrylic acid, styrene polymers and terpene polymers. The examples illustrate the spinning, stretching and dyeing of fibres according to the invention. Specifications 950,852 and 992,011 are referred to.