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32.3 parts of 4,5-dichloro-1-(N'-difluorotriazin-2'-ylaminoethyl)-pyridaz-6-one are dissolved in 150 parts by volume of N-methylpyrrolidone, and the solution is added dropwise at 40° C. to a neutral aqueous solution of 26.8 g of 1,4-phenylenediamine-2,5-disulfonic acid. The pH is then kept at 6 for 4 hours with a little sodium bicarbonate. The mixture is then cooled to 0° C. with ice and diazotized with sodium nitrite in the presence of hydrochloric acid, and the resulting diazo suspension is added to a suspension of the red monoazo dye obtained from 17.3 parts of aniline-4-sulfonic acid and 31.3 parts of 1-amino-8-hydroxynaphthalene-3,6-disulfonic acid by a method similar to that described in Example 1. Coupling is completed overnight at pH 5.5-6, and the product is precipitated with 250 parts of sodium chloride, filtered off and dried under mild conditions to give the dye of the formula ##STR55## in the form of a black powder, which dyes cotton in navy hues possessing generally good fastness properties.