Method for stabilizing brightened modacrylic fibers

The method of preventing color-degrading interactions between certain optical brighteners and filaments made from a polymer made up of 35 to 85 weight percent acrylonitrile, 65 to 15 weight percent of a halogen-containing ethylenically unsaturated monomer and up to 50 weight percent of at least one other monomer copolymerizable with acrylonitrile, said optical brightener being selected from the group consisting of ##STR12## where X and Y are basic groups selected from the group consisting of --NH.sub.2, --N(CH.sub.3).sub.2, and --N.sup.+ (CH.sub.3).sub.3 Z, where Z is selected from the group consisting of chloride, sulphate or bromide anions, R is an alkyl group containing 1 to 4 carbon atoms and n is 0 to 3, ##STR13## where R.sub.1 is a phenyl radical, R.sub.2 is hydrogen or an alkyl radical having 1 to 6 carbon atoms, a phenyl radical or a napthyl radical and R.sub.3 is a phenyl or napthyl radical, ##STR14## where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are hydrogen, an alkyl radical having 1 to 6 carbon atoms or a substituted or unsubstituted phenyl or napthyl radical, and ##STR15## comprising contacting the filaments with a bath containing 0.01 to 0.5 weight percent of an organophosphorous compound having the formula ##STR16## wherein R is a member selected from the group consisting of an alkyl radical having 1 to 11 carbon atoms, a phenyl radical and an alkyl radical containing no more than eight carbon atoms.